I think Rowling has something against thin people. Notice how all the evil wizards in her books are thin? Snape, Voldemort, Malfoy... I could continue but I think I've proved my point.
/sarcasm
Well, Voldemort is undead, so of course he's thin :-).
Mrs. Weasley, whom we're obviously supposed to like, is plump, which blows the "bad fat people" theory.