Why are all the A&O wolves so skiny?

It's just the way the decided to do it. And (I need to watch the movie again lol) Humphrey's fat friend was chubby. Also real wolves aren't really big either.

Well, in the movie the packs were struggling for food, so making them look emaciated kind of adds to that. It shows that they don't get enough food so they're all skinny.