Very simple I need some really big and imposing marines. The enhancements make them stronger with faster reaction speed, bone strengh and the works. Why would these marines be really tall as well? Is there any advantage to them being tall if they are deployed on space stations and ships?
Better Power Armor
Based on your previous questions, I will assume you have a setting that includes power armor.
One of the biggest complications in designing power armor is that you need a lot of space around the body to fit the exoskeleton and chunky armor plates, which leads to issues with articulation. The stockier your soldier is, the smaller spaces you have to work with for this; so, a tall, lancky soldier would put a lot more distance between your points of articulation allowing you to make your armor plates thicker and your exoskeleton stronger while still being able to move around your user.
In my previous answer to your question regarding the use of 50cal machine guns, modern body armor capable stopping such a weapon would have to be about to be about 3cm thick... doable but only in select locations. With longer appendages, you could design articulating plates than can be this thick around your entire body giving much better protection, and also give your space marines a tough enough of a powered exoskeleton to carry larger firearms that might punch right through the thinner armor of shorter soldiers.
As for your soldiers being breed to be overall "bigger" and "stronger", this part is wholly unnecessary. Machines are proportionally much stronger than biology; so, if you make your space marines tall and skinny, then you leave more room for robotic strength enhancements. Either way your space marine ends up the same amount of bulky in full armor, but by being thinner, more of that actual bulk is robotics and armor.
You want the most soldier for your money. Best intimidation factor, best carrying ability.
Training a midget costs exactly the same as training a Conan.
And training cost is the #1 cost in any military that uses professional troops, not conscripted cannonfodder.
Same for equipment, even to a partial extent clothing and armor. A sss-tiny soldier's boot costs just about as much to make as an XXX-Large.
And the Conan can carry a Vulcan Cannon over his shoulder while storming the enemy, while a midget struggles with a single-fire 12-gauge shotgun. Conan can carry a lot more ammo, withstand more recoil, hit harder in hand-to-hand combat.
Also, durability in combat.
Shoot the Conan in the arm with a 9mm round, and unless you hit a bone, he just charges on.
Shoot the midget with the same 9mm round, and his arm is off.
Yes the Conan is an easier target, which counts against him. Also more effort to feed, transport. But these are relatively minor matters.
Using Conans vs. Midgets to exaggerate the differences, but the same argument goes for intermediate sizes, just less obviously so.
To Reach Things
I'm reminded of Ender's Shadow/Enderverse Battle Room maneuvers. The gist is that kids at a space station military school play a 0-G game of laser tag in a box against other groups of kids. Bean, the smallest kid, has some problems with this as the handholds on the walls are spaced far enough apart that if he flies poorly he lands in a place with no handholds, and he bounces away from the wall instead of being where he needs to be.
Your super-soldiers could have similar considerations in mind when they're made Tall. Above and beyond the need for a bigger body to accommodate the extra muscle/fancy bones/what have yous, a physically taller body would allow for greater reach when it comes to grabbing hold of stuff in a 0-G environment. As long as they can still fit through ship's corridors with relative ease, you'd want them as tall as you can get. That way when fighting on a ship or station's hull, or in, say, the cargo area of a kilometers-long-freighter with no gravity, they'd be more maneuverable than a normal 5"10ish person. (or at least better able to get where they needed to go, maneuverable might not be the right word.)
Added to this "maneuverability" in 0-G, you also have the traditional intimidation factor that comes with fighting someone larger than yourself. It's instinctual and not to be underestimated when fighting in close proximity. The added height/dis-proportionally long limbs may also be an ID marker in the field. If your Space Marines are easily identifiable as such, AND are super-human killing machines, making them easily identifiable is another psychological advantage. For example, the Spartans had a specific symbol on their shields, and they sparked fear in their enemies. But in one battle against Thebes the Spartans used not-spartan shields and the Thebes held their own, whereas if they'd known they were fighting SPARTANS they may have broken and run. So if your Space Marines look physically different it'll let your enemies know they're up against the First Team and maybe give up before the fighting even seriously begins.
While working on other traits such as efficient use of oxygen and stronger muscles, they found it had the effect of increasing the size of the soldiers.
Sure, they can sing the praises of the intimidation factor and the ease of fitting power armor, but had it been the other way round, they would be singing the praises of lower calorie demands and decreased chance of cancer.
Born in 0 g.
It turns out humans born in zero gravity get really tall. Unmodified children can grow up to eight feet tall and sometimes parents of children destined for civilian life will induce puberty early in order to halt growth at manageable heights between 6 and 7 feet. The really tall ones struggle if they are ever in a situation with gravity because generally in addition to their height they have low muscle mass.
0G spaceborn children chosen to be space marines are often allowed to reach full height. Enhancements augment muscle mass to some degree but these persons are still lanky as compared to persons born in a gravity well. Really, strength does not matter if you have on power armor which they invariably do if they are working or on a planet surface. If power armor presents poor optics for a given situations, the spaceborn marines planetside will still wear a light exoskeleton frame.
Because the dictator who ordered them is really short and is trying to compensate.
Being taller comes with spine issues, it's actually an advantage to be a little shorter when hauling the sort of loads soldiers deal with, this applies even more so to special forces.
The palace guard should of course be really tall.
Two possible reasons: intimidation and the ability of moving more quickly (longer legs means higher speed e.g. when running).
It could also be a necessity if they need space to store synthetic enhancements (e.g. oxygen reserves, food supply, chemicals) that can't as easily go in the outer armor.
Bigger usually = better in combat
There's a general observation in nature that bigger usually means better when it comes to combat between animals. Bigger animals can more easily win fights and monopolize resources compared to smaller ones. Size generally provides a multiplicative advantage because while size (in humans usually measured as height) increases linearly, weight increases cubically due to the square-cube law. What this means is while a 6 foot (183 cm) human might weigh 200 lbs (90 kg), an eight foot (244 cm) human will weigh 215 kg (474 lbs).
These differences in weight are a big deal. They're the reason why you see weight classes in sports like wrestling, MMA fighting, or boxing. This is because pound for pound, assuming a fair fight and evenly skilled combatants, the larger combatant will win most of the time. Smaller people can win by being more skilled, having better weapons, or using better tactics (element of surprise, etc.), but they won't win in a straight-up fight.
Larger size would also mean that your super soldiers could carry heavier gear or wield larger caliber weapons than regular soldiers. Two of the biggest constraining factors in modern soldiers are long-term fatigue in carrying the very heavy backpacks and gear that most soldiers have to lug around, and the weaponry they can carry is limited by how much the human body can easily lift. This is why in science-fiction franchises like Warhammer 40k or Halo you see super soldiers wielding weapons that are handheld rocket-launchers or particle cannons, or even outright lifting an emplaced heavy machine gun from the ground and wielding it as if it were a minigun.
Larger size also means it takes less energy per pound to move from one location to another. Relative to body length, migration is much less of a big deal for an elephant than a mouse. This is why a lot of migratory animals are very large. This might also mean your super-soldiers have an easier time on long marches.
There are distinct disadvantages to being larger in combat.
- Bigger soldiers are larger target for firearms
- Bigger soldiers require more calories to support and if you can't feed them won't be cost effective
- Bigger soldiers may not be able to squeeze into enclosed spaces like sewers or tunnels to fight in asymmetrical warfare
- Super soldiers can't go undercover. A seven-to-eight-foot-tall supersoldier can't easily pretend to be "one of the locals".
But a lot of these aren't absolute deal-breakers if you can logistically deal with them. If you're looking for reasons for them to be big, here you go.