August's true form exists in a pocket of extradimensional space - the usual shape, with the four arms, four legs, and wings is just the smallest one he can exist in without falling out of a world entirely. He can push more through if need be, though, and there doesn't seem to be a limit. If he needs (or wants) twelve heads and fifty arms, he can do that with no problem. There's no problem with internal anatomy, either; all of his actual vital functions are handled back in the pocket dimension most of his body lives in. The August that most people see is only a small piece of a very large whole.