As far as I can tell, all baby animals have the same hitbox size as their adult counterparts. I don't know how this could be reasonably fixed code-wise, considering how Mo'Creatures mobs grow, but it can make some weird effects... For instance, ostrich chicks that have just hatched from eggs can suffocate in a block that's above their model's head, and shooting over their head will shoot the ostrich. Same thing for BigCats and manticores (and every baby animal, but it's most egregious there).

Well-known issue but we're unsure how to tackle this currently, we might emulate vanilla's EntityAgeable mechanisms.