Body image isn't always accurate. Surely you have heard of anorexia?
Clip from white chicks about that (opposite of a fat person thinking they are not, but similar principal I guess):
http://www.youtube.com/watch?v=6Gy4xMgArmk (dressing room scene -- possibly NSFW)
Clip from white chicks about that (opposite of a fat person thinking they are not, but similar principal I guess): http://www.youtube.com/watch?v=6Gy4xMgArmk (dressing room scene -- possibly NSFW)