both limb and body length, mainly. I mean, a hamster which stands with a hand touching the ground is imho a gorilla who put a hamster's mask on. (no offense intended).
As i scan the 4 characters, i could initially not tell that zilla and werewolf used the gorilla template (though changing sizes might be a good idea there too), the hamster just couldn't sound right and revealed the truth for both 4 guys: re-colored gorillaz with another head.
I know of deadlines, engines restrictions and so on, but what about making the hamster actually have the shape of a hamster, the zilla having a larger tail (to keep balance) and maybe have each character having its favourite attack (zilla might give tail shots rather than punching, etc). That would make the game more interesting as the alternate characters would mean a (slightly) different gaming experience. Think of it as how flat Street Fighter would be if all characters had the same abilities.
(just 2 cents)