P.M.
It is fascinating and disturbing to hear that from such a young child. It sounds like your attitudes about body size are healthy. Hard to say sometimes where these thoughts originate.
I would be careful not to insist that she's wrong, because that could cause her to cling more tightly to her opinion, thinking that you just don't understand. There's a great book that can help you navigate this issue and the many more you'll face in the coming years, called How to Talk So Kids Will Listen, and Listen So Kids Will Talk. You can learn how your children can propose their own solutions to a mind-boggling range of problems. Read part of this wonderful parent-workshop-between-covers here: http://www.amazon.com/How-Talk-Kids-Will-Listen/dp/038081....