# What conditions might create bigger military submarines?

I'm creating a story set on a near-future Earth in which undersea colonization is fairly widespread. I want large (think undersea equivalent of a battleship) and, especially, expensive submarines to experience extreme military advantages over larger numbers of smaller, cheaper subs. Ideas I've considered:

• Advanced armor
• Widespread use of EMPs and signal jamming

Would either of these help to explain such a military condition? What other technological conditions might?

• A submariner might prefer to think that submarines are 'naval vessels' and not 'military' (which sounds too much like the Army). – a4android Aug 7 '17 at 2:31
• Does the surface analogy for these big subs have to be the battleship, as opposed to the aircraft carrier? – Scott Aug 7 '17 at 4:42
• There was a bit of a trend for this at the start of WW2, e.g. en.wikipedia.org/wiki/French_submarine_Surcouf : cruiser guns, small aircraft hangar, although not a true submersible carrier. The main downside of submarines is they are even more vulnerable from the air than surface ships. Maybe you need a rubric like "climate change has caused extremely high and unpredictable surface winds making the use of combat aircraft at sea (especially carriers) impossible" – pjc50 Aug 7 '17 at 9:24
• =Starwars in an ocean. They're building a new weapon, I heard they call it the Deathsub, which can destroy whole <strike>planets</strike> islands. But! it has one vulnerability (people died to find out what it is, etc etc) - you can destroy it if you ram an even larger craft into it :-) – Nahshon paz Aug 7 '17 at 12:30
• @a4android Do you have any evidence for that claim? In the conventional use of the words, the army and navy are two branches of a country's military; the word "military" doesn't seem to imply "army" at all. – David Richerby Aug 7 '17 at 15:13

## 19 Answers

Sadly bigger won't mean better.

Bigger subs will be less maneuverable than smaller subs and thus prone to flanking and hit & run tactics (see pirates).

An additional advantage of many small subs over one big subs is that a single big sub is easier to hit. A group of subs can split, scatter and reform easily and thus fare much better in a fight.

So why would we still favour bigger subs?

Smaller subs will have to fill way more specialized roles. They cannot carry lots of fuel, armor and armaments. Bigger subs can be much more complex and versatile, though the disadvantages mentioned still hold.

If we pull the bigger subs back from the frontlines we can get some better result. In modern naval warfare aircraft-carriers play an important role as the backbone/center of a strikeforce. They carry ammunitions, fuel and other things while also providing a base for refueling and parking smaller limited strike-craft.

So we can make the big subs into sub-carriers that each support 2-3 groups1 of smaller hunter subs. The big carriers can be heavily armored as we don't care much about weight - after all, we want them to sink!

Thanks to the square/cube law we have more material available for armoring the big sub than if we were armoring the same volume of smaller subs.

When attacking they would likely send out two of their three groups, keeping at least one group back for defense purposes. If you are fine with having them fill in more stationary roles you could easily scatter minefields around their position, perfectly safe for friendly subs but deadly for any attackers.

What if I want them to be even bigger?

Additional size demands purpose. If you can find reasons why they need to grow bigger, then go for it. Just remember, additional size requires additional resource in form of materials to build it and technicians to maintain it - any unused spaces still have a cost attached to them!

One possibility is to make the surface world inhospitable due to whatever you choose. Force people to go underwater. As the military needs lots of support personnel, they will take on civilians and sections of the bigger subs will turn from living quarters into little villages.

In time those subs will grow with e.g. additional influx of refugees. Little city districts will form in the inner sections of the subs, surrounded by factories and machinery which is in turn surrounded by the military forces it is built to support.

1Well, as many as you want to be fair. The only limiting factor is the size of the sub and the kind of docking facilities you wish to implement.

• What about... a large sub built to move hundreds of smaller subs to be deployed in a different location? – Cameron Leary Aug 7 '17 at 6:55
• Heavily armored means they can go deeper, and a larger sub has less surface area per internal volume, so can have a thicker hull. In fact, it also has a smaller cross-section per volume, so could possibly be faster than a smaller sub? – JollyJoker Aug 7 '17 at 8:09
• @CameronLeary sure. Just make it bigger. Although at a certain point you will be moving over to a catamaran-like design due to the rules of physics and pressure containers. – dot_Sp0T Aug 7 '17 at 9:38
• @JollyJoker on a straight indeed. The issue is maneuverability. The bigger - the longer - thus less maneuverable compared to smaller ones. Make the experiment, try to rotate a desert spoon and a cooking spoon in the kitchen sink (filled with water naturally). – dot_Sp0T Aug 7 '17 at 9:40
• @dot_Sp0T Documatairy: youtube.com/watch?v=kEdu2avMDQI Wikipedia: en.wikipedia.org/wiki/I-400-class_submarine – Jeroen Aug 7 '17 at 13:04

Hmmm.... As you increase the size, volume goes up faster than surface area; a 1 unit cube has a volume of 1 and a surface area of 6, a 2 unit cube has a volume of 8 and a surface area of 24, a 3 unit cube has a volume of 27 and a surface area of 54, and so on. That means that if you want really thick, heavy armor, you can make it a lot thicker on a big sub than on a smaller one before it starts causing bouyancy problems. So that's one possibility.

Alternatively, you could look at the underwater equivalent of aircraft carriers. Maybe have more effective attack subs (very fast super-cavitating ones, maybe?) that are a lot more combat effective than normal subs, but have limited endurance (like fighter planes) and need the equivalent of a carrier to supply them and for the crew to live in. (I'm assuming that only having to pack enough life support/fuel/supplies into your fighters to keep them going for several hours to a few days should let you make something more effective than something that needs all the support and fuel for several weeks or months...)

Or some critical piece of equipment (highly effective weapon, defense, or whatever) that is sufficiently large that it needs a big hull to fit it in.

Bigger submarines are for longer missions.

Submarines are thought for stealth, big and stealth do not go well together.

The only sensible reason to make bigger ones is to stuff into them more weaponry and recycling equipment to allow remaining deep and quiet through the rough moments and strike back with enough power. In general nuclear submarines (nuclear in both power and weapons) are much bigger than "conventional" ones. You need much more space to hold the nuclear reactor and you need to transport a sizable amount of ICBMs. To that You need to add space for "enhanced" technical staff.

Again: in submarines big is for longer time underwater.

• Current nuclear submarines already have practically limitless endurance, so no need for them to become even larger. Unless you are talking about decades-long missions... – vsz Aug 7 '17 at 6:20
• @vsz limitless only in terms of fuel (and there limitless is 10-15 years in practice). Crew rations, toilet paper, deodorant, weapons, coffee, all are carried for only a few months at most. Which is about the time after which cabin fever will set in and morale problems get bad, the crew has become the limiting factor on mission duration rather than the ship. Though after half a year to a year at sea the spare parts situation will be kinda bad too and things start to break that become ever more critical to the mission. – jwenting Aug 7 '17 at 7:41
• @vsz I fully agree with jwenting. I specifically spoke about better recycling and maintenance while I forgot to explicitly mention "less cramped living quarters", which are more important for long term missions (I am speaking about ~1year) than sheer energy which current nuclear submarines have aplenty. – ZioByte Aug 7 '17 at 8:34
• @jwenting : yes, a few months, normally. In an emergency, and with adequate preparations, I'm pretty sure they could manage for years. Astronauts have much less space and they managed to stay in tiny capsules for very long time periods. – vsz Aug 8 '17 at 6:05
• @vsz very long periods? The longest missions (not counting Mir and ISS which get regularly resupplied from earth, and have crew rotation schedules) were 2-3 weeks. Those were the Apollo missions to the moon, btw. Every other manned mission typically was over in a few days (some Shuttle missions might have lasted a week or so, but they had plenty of room to walk around and store supplies for that long, the Shuttle was quite roomy). – jwenting Aug 8 '17 at 6:23

Active sonar may make it easier to find a larger sub (absent shielding), but underwater, most subs are going to be using passive sonar (listening for sounds)

You could go with the "Red October" solution, stolen from the Tom Clancey novel. Namely, a more silent propulsion drive requires substaintially more area. Red October used some fancy new system, but, in general, bigger propellers are quieter.

Acoustical shielding may be thick, and in general expensive. That would let you benefit from the square-cube law.

Other systems, e.g. nuclear reactors, may also get more silent as they scale up. That can certainly be posited in a reasonable way.

• Words I never thought I'd hear on worldbuilding SE "benefit from the square-cube law". – Gryphon - Reinstate Monica Aug 8 '17 at 4:01

A size comparison might come in handy for starters.

Firstly, the world's biggest submarines.

Typhoon Class, Russia

The Typhoon, with a submerged displacement of more than 48,000t, is the world's biggest submarine class. It is a nuclear-powered submarine equipped with ballistic missiles. Dmitry Donskoy, the first of the six submarines in the class, was commissioned in 1981 and is still in active service with the Russian Navy.

Typhoon Class submarines have a length of 175m, beam of 23m and draught of 12m.

Borei Class, Russia

The Borei Class, which has a submerged displacement of 24,000t, ranks as the world's second biggest submarine. It is a nuclear-powered missile carrying submarine serving the strategic naval forces of Russia.

Borei Class measures 170m long, has a beam of 13.5m and draught of 10m.

Ohio Class, US

The Ohio Class submarine is the third biggest in the world. The US Navy operates 18 Ohio class nuclear-powered submarines, which are the biggest submarines ever built for the US. Each sub has a submerged displacement of 18,750t.

Each Ohio Class submarine has 170m length, 13m beam and 10.8m draught.

Secondly, the size of battleships. This is an example of a modern battleship. The dimensions for an Iowa-class battleship are: --

Type: Battleship Displacement:

45,000 long tons (46,000 t) (Standard)
52,000 long tons (53,000 t) (mean war service)
57,000 long tons (58,000 t) (pre 1980s full load)
58,000 long tons (59,000 t) (post 1980s full load)


Length:

861.25 ft (262.51 m) pp
887 ft (270 m) oa


Beam: 108.2 ft (33.0 m) Draft: 36 ft (11 m) maximum

Essentially a battleship-sized naval submarine would have, roughly, double the length of the current biggest submarines. two hundred and seventy metres versus one hundred and seventy-five. This suggests that scaling up submarines to battleship may not be implausible.

One consideration is that in a world where undersea colonization is a feature. Submarine freighters and tankers might also exist. While it does make sense that smaller, faster submarines would be better placed to defend and protect such civilian submarines. It's not infeasible that battleship-sized submarines could be employed to take on a similar role to that of battleships. Namely, as a major support unit for a submarine battle group.

• For active service, apparently specifically the Dmitry Donskoy paid a visit to the neighborhood of Denmark just a few weeks ago. – a CVn Aug 7 '17 at 13:59
• @MichaelKjörling Reds on the seabed. An uncomfortable thought. I wouldn't want a vessel like that in my neighbourhood. – a4android Aug 8 '17 at 2:14
• Note that you can not compare displacement numbers of submarines and regular ships if you use them as a measure for size. A submarine displaces its whole volume of water while a surface ship only displaces the water below the water-line. – Philipp Aug 8 '17 at 13:36
• @Philipp Quite so.The comparison of sizes was by length and beam not by displaced volume. – a4android Aug 8 '17 at 13:40

Large submarines in general are more difficult to hide, are difficult to manoeuvre and will have difficulty operating in littoral regions. The only real reason that you might want to have a much larger submarine is to have a long endurance platform which can use much more of the ocean to hide in.

This means combining two separate and not entirely ompatible goals. You want a large submarine which can hold stores and supplies for long endurance cruises (or large numbers of weapons, essentially an arsenal submarine).

OTOH, in order to truly effectively use the ocean to hide, your submarine should be capable of diving much deeper than conventional submarines, so the crush depth might be well past 500m. A sub which can dive to 1000m can effectively sit on the bottom of much of the continental shelves and be relatively safe from detection hiding in the "clutter" of the ocean's bottom, or alternatively hide under multiple layers of salinity and temperature, making detection much more difficult (dipping sonars to breach the thermoclines would now need to be suspended at least 1000m down to attempt to find the submarine, and it could be at almost any depth between the surface and its crush depth.....

A large submarine which is spherical in shape, rather than the conventional "cigar" shape will be able to fulfill many of the desired parameters. A sphere has maximum volume to minimum surface area, is immensely strong and the spherical shape is probably good for ensuring active sonar does not get any clear return (outside of coating the hull with metamaterials to refract active sonar around the hull).

A large sphere isn't going to be as fast as a more conventional hydrodynamic hull shape, but this is one of the trade offs that will need to be calculated. As an aside, the USN and the Indian Navy now have boats the size of conventional attack subs which have much of the firepower of larger and more expensive "boomers" (Virginia class and Arrant class respectively), demonstrating that bigger isn't always better.

USS Virginia

Arihant

As a different option, what about ultra-long-haul submarines? Nuclear submarines' propulsion is effectively unlimited (at least within the lifetime of the submarine), but they are limited by their ship's stores.

So if you had several decks devoted to hydro/aquaponics to supplement what could be harvested from the ocean, you could have a submarine that could potentially operate for years without a resupply, and on which the operators could continue to eat healthy meals. This would, by necessity, need a lot more space, and would be more like a roving habitat than a battleship.

Depth could be a reason, think of a tin can submerged to a mile deep versus the same tin can made from fifty foot thick titanium alloy, the first would implode whereas the latter may have enough strength to deal with the pressure. Thats the most plausible solution I could think of to explain why one submarine was larger than another. This could be tied into the need to submerge to lower depths than smaller enemy submarines.

Other reasons could include:

• A larger population size
• Evolution of predators requiring counter evolution (which as we know, in humans, tends to translate to more advanced and/or lethal technology)
• Storage of resources, which also ties into larger population size
• Whales develop a second stomach for the digestion of submarines, ties back to the predator scenario
• Invention of fusion technology allowing submarines to be fueled by stars, requiring larger subs
• Use of nuclear offensive requiring thicker hulls to sustain greater levels of radiation
• Acidic seawater due to pollution and requiring more heavily armoured subs with much better water filtration systems
• Fatter population because they aren't getting enough exercise at the bottom of the sea
• Requirement of solar farms to sustain a population that has never seen sunlight above the waves

Theres a thousand and one different ways you could go with this honestly!

• Unfortunately, pressure works in the opposite direction: It forces small size. The smaller the radius of the hull, the thinner its material can be to withstand the same pressure. I believe, that there are exactly two reasons for the big sized nuclear submarines that we see today: 1. Nuclear reactors are big, and 2. nuclear warheads are big. Both come with certain minimal sizes, which require supportive systems (cooling for the reactor, rockets for the warheads) of certain minimal sizes, so the sub has to be big enough to hold it all. I guess, modern subs are quite close to the size limits. – cmaster Aug 7 '17 at 18:41

Changes to underlying technologies would naturally lead to larger submarines. Specifically available hull materials and reactor and ballistic missile technology would have the largest impact.

## Hull Materials

Availability of cheap supplies of titanium and familiarity with alloying and working with the materials could allow much larger submarines to be built. In cold war history, the Soviets produced several submarine classes with titanium hulls due to the greater supply of titanium in their territories. This includes the Typhoon Class the largest class of submarines build to date (it had a pool inside it!).

## Reactor and Missile Tech

If miniaturization of submarine nuclear reactors encountered difficulties or alternate technologies evolved which needed very large amounts of power, then a larger less efficient nuclear power plant would require a larger submarine superstructure to support it.

Similarly if ballistic missile tech was less advanced, or other conditions existed (less advanced larger atomic bombs), which required larger missiles then the submarine would again need to be bigger to allow the same military functionality.

If under-sea colonies are common, then the purpose of at least certain types of submarines could correspond more to their above-water counterparts (for example, cruise liners for underground colonies for non-military applications).

For military (and even standard business) applications, think "troop transports" or "cargo ships." Just look up pictures of current cargo planes or oil tankers - they're enormous. Presumably, you'd still need those things underwater, which would require large submarines to accomplish. (Current submarines aren't even remotely suitable for that kind of a purpose). The underwater equivalents of trucks and trains would be nice to have, too. In fact, I seriously doubt that you could sustain any kind of a large underwater colony for any length of time without some economical way of moving large amounts of goods and people at once.

A few other things that could cause that:

• Colonization of deeper parts of the ocean.
• Slightly larger planet leading to more gravity, which necessitates thicker submarine "skins" to cope with the extra weight

For "standard" fighting-type applications, though, you'd probably want to stick with smaller, lighter submarines. (Think about how comical it would be to try to have a dogfight with 747s - unless you want comical. If you want to include some humor, you could actually have someone try something like that and have it flat-out not work). A better application fighting-wise would be if submarines were mostly just attacking large ships.

If global warming melted all the icecaps (Water world) such that there's no longer much livable land, Kevin Costner might prefer to live in a giant city under the sea. Such a city would have the advantage of not having to withstand the elements of the open air.

However, changes in the underwater environment would require that it move from place to place, from time to time (e.g. foraging/farming/mining grounds, seasonal areas of cold/warm water, enemy incursions to their area).

Now that they're on the move, albeit a very slow move, they ought to be able to defend themselves, particularly against other city-sub-states that are also out to find good places to mine/farm/do the "Under the sea" song.

Soon it'll be a red ocean, where gigantic submarines containing thousands of people each are battling it out with "Under the sea" in the background.

The horrors of war.

## First, can you make super-sized subs?

Your giant submarine will require larger engines, more materials, and a bigger crew. Can your fictional world supply all that?

Can it accept the loss of maneuverability that comes from a larger vessel? The risks of detection that increase as your vessel's surface area increases? The higher fuel and supply costs?

## Some reasons to go up in size

1. Dive depth. It might be that a larger sub can be built to dive deeper by having a stronger structure and a thicker / tougher hull.
2. naval supremacy. Present day warfare favors air supremacy over naval. The focus is on drones, bombers, and other aerial attack vehicles. Battleships are obsolete. So your world needs a reason for naval forces to rise back to the forefront of military technology (and spending), and for that focus to be on subs, not other surface ship types.
3. Surface conditions. If the surface is no longer safe, then subs become more useful.
1. Biological. If a plague (man-made or natural, zombie or mundane) breaks out, then a sealed vessel below the surface might be safer.
2. Global warming. If the surface is too hot, or subject to too many hurricanes or other violent storms, then subs become safer.
4. Surface threats. If your world's technology has advanced to the point where surface vessels have extremely low odds of survival, then subs become stronger alternatives. Think of the German U-boats and how devastating they were early on during WW2. Now ramp that up. Or an alien invasion that uses a sustained meteor shower to bomb surface targets. Or... You get the idea. If all your ships must be subs, then your subs might need to be big enough for roles now carried out by other vehicles (troop transport, supply runs, etc.)
5. Energy surplus. Maybe cold fusion becomes a reality. Or some other exotic power source. But your big sub can now have all the power it needs, so why not go big?
• Dive depth works against big sizes: The smaller the radius of the hull, the more pressure it can withstand with the same material thickness. – cmaster Aug 7 '17 at 18:47

On the physics:

If you take a sub, scale all its dimensions up in the same way, the resulting bigger sub won't be able to dive as deep as the original one. That is because you are decreasing the radius of the hull as you are increasing its size, and it is the curvature of the hull that allows it to withstand the water pressure.

Obviously, you can still increase the size of a sub by simply joining several subs together. Likely, those bound-together pressure cylinders will be enclosed within a thin, non-pressure withstanding hull for hydro-dynamic reasons. Each pressure cylinder, however, will have a rather small diameter, to allow for large diving depths.

How to explain gigantic subs:

Well, you obviously need a reason for large subs. As a matter of fact, subs are relatively poor at actually controlling an area (or water volume). To do this, it might be necessary to use a large fleet of small subs. However, small subs cannot use nuclear reactors, as those have a certain minimal size. So, the small sub fleet would have to do with battery power. But how can you reload your batteries when you are on the other side of the planet, and want to avoid surfacing for whatever reasons? Enter the big sub carrying sub!

It would work a bit like aircraft carries today: You have one really big vessel, which contains at least a dozen small vessels, which are the big vessel's eyes and arms. These small vessels patrol the area that is to be controlled, and they strike against any enemy they find within their action radius. Once their batteries are down, they return to their carrier for a battery charge, remunitioning, and some fresh supplies.

The carrier sub itself would be huge by todays standards. It would likely contain several powerful nuclear reactors, large ammunition storage areas, and have several places for the small subs to dock. The parts that need to be at atmospheric pressure would all be contained within a number of long cylinders, each no more than five meters in diameter for structural efficiency. This pack-of-thin-cylinders design would go quite well with its purpose of being a carrier of many small subs, I think.

The big sub itself would not have much defensive capabilities itself, relying on its fleet of small subs to defend it, just like aircraft carriers rely on their air force to defend them. The big carrier sub might contain a few big offensive missile type weapons, though; weapons larger than the small subs can carry.

Such a carrier sub would really be able to control a certain volume of the sea, which is hard to do with the modern subs which are mostly built for offensive strikes on the surface. The modern subs are built that way, because we humans are most interested in controlling the earths surface, for obvious reasons. A civilization that mostly lives underwater would have much different priorities: They would desire to control either water volume, or areas on the bottom of the oceans, so I think, big carrier subs would suit them well.

A major breakthrough has been made in stealth technology. The new technology makes a submarine totally invisible to even the most advanced sonars/radars/cameras/etc.

Problem: This technology requires a yehudicollider, which is a very large machine that can't be miniaturized.

Well, some simple answers could be that the pilot race is bigger - perhaps there's a need for more people on board, not just to pilot it, but maybe even to live and work on if it's needed.

Or, perhaps, the sea is bigger and the vessels need bigger bulk out of the materials they're made out of in order to survive the sea conditions? I'm not very experienced with undersea stuff, so these are pretty pedestrian ideas from me I'm afraid.

Additional considerations... perhaps each ship shape works like a 'key' and they need to be a certain size to get back into the port area or stations? Like, the door wouldn't open for them if they were too little or too big, and maybe these 'shells' are adjustable for certain configurations.

Or, of course, maybe there's just a need for bigger and bulkier ships in general - maybe as intimidation against natural beasts/predators/foes?

## Drones & Missles / Troopship / Supply & Repair

Think of the ocean as deep space. A larger ship has many advantages.

A large, deep diving vessel that could launch massive numbers of underwater drones, conventional missiles, nuclear missiles and stay out of the way by going very deep would be very useful - especially if it could raise from a deep trench, launch massive drones and missiles and then disappear again.

Additionally, a variant ship could launch amphibious landing craft from deep in the ocean and disappear - providing a profound element of surprise (troops coming out of the ocean) - especially when backed by the battleship variant.

Another variant would also be large mobile Supply & Repair ship that stays underwater and provides assistance to the fleet (c.f. https://en.wikipedia.org/wiki/USNS_Lewis_B.Puller(T-ESB-3))

These capital ships would then form the backbone of an underwater fleet that takes advantage of being hard to detect, hard to track and hard to destroy.

There is one reason which may actually become an actuality fairly soon. By covering the surface of a submarine with 'pixels' of materials that facilitate detection of neutrinos (eg: conversions to muon, Cherenkov radiation...) it will be possible to identify the direction of neutrino travel by analyzing the entry and exit spots. This will allow communication with nuclear submarines from base stations without the need for 'dead man's handle' radio signals, or low-bit-rate extremely low frequency radio (radio does not penetrate salt water well). These submarines will have the advantage of advanced command and control. Another advantage is that other nuclear submarines emit neutrinos from the fission reactors, which is unstoppable, and detection will make them visible.

Use your fave search engine to find the above is under active research. https://arxiv.org/abs/0909.4554 https://arxiv.org/pdf/0909.4554.pdf https://tech.slashdot.org/story/15/10/13/1727231/antineutrino-detection-is-about-to-change-the-game-in-nuclear-verification By the way, China will be deploying ultra large neutrino detectors in the near future.

• If you could provide references it would be better. Borexino and other neutrinos detectors use ultrapure water in complete darkness to detect the fain scintillation produced by neutrinos interacting with matter. Both conditions are not met in open sea. – L.Dutch - Reinstate Monica Aug 7 '17 at 13:16

Bigger torpedoes have a longer range (possibly even over-the-horizon), and bigger submarines can carry these bigger torpedoes.

Bigger submarines carry more torpedoes (real-life WW2 submarines carried only 6 to 24 torpedos).

Bigger submarines might also carry aircraft; or carry whatever the underwater equivalent of aircraft might be (e.g. fast drones); or carry inter-continental or surface-to-orbit missiles.

Bigger (hypothetical) submarines might also carry its own defenses ... not armour but for example deployable mines, so that it can lay a mine field (and/or sonar listening devices) around itself ... so that it's a mobile fortress. Or, miles of some kind of net.

More/bigger weapons

Battleships were created because they could carry bigger cannons. So maybe bigger submarines could launch bigger torpedos that have better chance to deal enough damage to score a kill. Or bigger size allows for better engine/more fuel meaning faster speed and longer range.

Death Star from Star Wars movies was so gigantic because it's main task was to house a huge special weapon with enough firepower to destroy planets. Modern big submarines are mostly that way because they are meant to carry and launch intercontinental ballistic missiles. So maybe there are some special weapons that simply wouldn't fit on smaller subs.

During age of sail bigger ships meant more firepower because they could carry more cannons. So bigger submarine could have more torpedo launchers leading to being able to strike more hits on enemy or being able to target more enemies same time. Maybe there are some anti-torpedo devices capable of destroying/jamming launched torpedos and more torpedos means bigger chance some of them will survive to score a hit.

Size requirements/bigger is better

I already mentioned specialized weapon that wouldn't fit small submarine but there would be other things that can have minimum size requirements- power source (nuclear reactor), engine, detection (special radar, sonar), defencive devices (energy field generator), communications (special long range radio) etc.

Bigger submarine would mean more room for stuff like supplies that would mean longer operating range. Or more people like marine detachment for doing and defending against boarding actions.

Bigger submarine could also support improved versions of things like thicker armor.

Carrier

Today the biggest military ships are aircraft carriers so maybe submarine version of those. They can carry normal planes or small strike subs.