Small vs. Large! If you had nearly the same creature, and the only difference was size, what advantages and disadvantages would large size vs. small size have--beyond the usual--reaching leaves more easily and whatnot?
Large body better than small for:
- Intimidation of other species. A gazelle at the water hole is going to give way to an African buffalo or an elephant.
- Intimidation of your own species, in order to get laid. If males are competing physically for females, being bigger and stronger than your rivals helps you to win the fights and get the girls. This may drive the evolution of some species towards becoming bigger.
- Too big to eat. Because of your sheer size, the smallest predators can no longer attack you. Either (a) you are too difficult to kill (if a leopard wants to throttle a giraffe, it'll have to climb up to the giraffe's head to do so). Or (b) the amount of effort needed to kill you is not worth the pay off (some lions kill elephants but for most prides there are far easier prey. And (c) there is just too much of you to eat before a horde of scavengers turns up and huge fights break out - basically the predator puts in all the effort and lots of rivals get the benefit of the kill. Many more rivals than if a lion had killed a wildebeest rather than an elephant.
Smaller body better than large for:
- Maturing fast and getting started on breeding. Big bodies require more food and more time to grow. So smaller species will churn out the babies faster. (Partial exception to this rule: sauropod dinosaurs laid eggs about the size of an ostrich egg. So they were huge like elephants, but once they got to sexual maturity they could have squillions of babies, like rabbits).
- A more athletic body, so a more diverse range of possible lifestyles. The bigger you get, the more you curtail your options for locomotion, because you eventually exceed the safety factors that your skeleton can take. Horses can walk, trot, gallop and jump over fences. Rhinos, being heavier can walk, trot and gallop, but can't jump over fences. Elephants can only walk. When an elephant 'runs' it is really speed walking. If it tried to trot or jump it would break its legs. Same goes for flying - really big things can't get airborne.
- You can fit your body into all sorts of nooks and crannies. Wolves can give birth in an underground den, birds can nest in hollow trees, and so on. When you reach a certain size these options for hiding from predators and hiding from the elements are lost to you.
- Hibernating is dead easy, even if you are warm-blooded. Lots of teeny things from hummingbirds to hamsters hibernate, aestivate or go into torpor. Bears are the only large mammals to have cracked it, and some scientists say bears don't have 'true' hibernation because their body temperature doesn't drop very much. The reason it doesn't drop is because the amount of fuel the bear would need to raise it again to normal is unfeasibly large. Think of the energy difference between boiling enough water to fill a teacup (hamster sized volume) and boiling enough to fill a bath (bear sized volume). Obviously, things get more complicated when you include cold-blooded creatures like crocodiles.
Thermal insulation is another advantage, the bigger and rounder you are the easier it is to maintain your core temperature and warmer bodies are able to metabolise faster, also a lot of pathogens cannot survive out of a surprisingly narrow temperature range, you might even heat stress certain parasites too.
There are many animals species that are different in only size. The major difference is the prey/predator relationship. What can it eat? And what can eat it?
The larger an animal becomes, the fewer predators it has to fear. Obviously, the drawbacks of being large means it has to eat more and that requires more area. More area means more competition for food, as well as moving into the area of other predators.
The smaller an animal becomes, means there is more food for it to eat. Less mass, means less energy is needed to fuel it. Obviously, the smaller an animal is, more predators are able to eat it.
Advantages of a larger body:
- Strength large body means larger muscles.
- Stores larger body means larger places to store energy as fat.
- Intelligence larger body means more space to store larger brain.
Advantages of a smaller body:
- Requires Less resources to maintain.
- Can move more quickly.
- Can hide from predators in small spaces
A large animal can have a complex gut that holds a large volume and takes a long time to process a batch. The sauropods got big, in part, so they could efficiently digest the folage available at the time. Basically it contained a huge fermentation vat.
To paraphrase JBS Haldane: "Drop an insect down a mineshaft and it will flutter to the bottom. Drop a man and his bones break, a horse splatters."
Haldane wrote the definitive article on the subject called "On being the right size". He also pointed out the role of the square-cube law which explains why giants don't exist. It can be found here and there is also a link to a copy in downloadable PDF format.
There is also a relationship between size and ecology. Dinosaurs roamed around on very large landmasses courtesy of continental drift. The remains of Mammoths were discovered that lived on islands and had evolved into pigmy species. Big animals need large areas for their ecology to support them. The smaller the ecosystem, the smaller the organism.
An aside: considering King Kong and the dinosaurs he fought lived on Skull Island, if normal biology and evolution applied then King Kong would have been a pigmy giant ape and the dinosaurs only the size of chickens. This means King Kong would be around the size of a teddy bear. Somehow I can imagine a scene with Fay Wray holding pigmy King Kong in one of her hands. :)