C.C.
Well... you can't blame the child. This is what our whole society thinks, whether or not they admit it. We're completely obsessed with being thin, while we as a society continue to get bigger. You can't turn on the TV without being bombarded with ads about how to get thin without trying, and how fat is ugly, etc. It's on billboards and there are reality TV shows where people compete to lose weight so they can be called "The Biggest Loser." It's EVERYWHERE, that's where she got it from!
With my own kids - one of whom is stick-thin and the other who is a sturdier kid - I try very hard to emphasize being healthy and fit, not thin. I talk about the kinds of food we eat, and why. I don't say that butter makes us fat, but rather that olive oil is a healthier choice for our hearts. When we're hungry for sweets, we eat fruit because it has the fiber and vitamins that we need. We walk a lot because that builds healthy muscles and helps our bones stay strong. In other words, I don't focus on what our bodies look like, but how we should treat our bodies with respect and take good care of ourselves. I try hard to praise their schoolwork and art work rather than praising them for being pretty. I try really hard not to make any negative comments about my own body.
I guess time will tell, but I am hoping that by emphasizing health, and not body size, that my girls will grow up to value fitness and will want to be strong, rather than striving for a certain shape.
Kids do comment on anything they find unusual (once my oldest made a very loud comment in a crowded store about a man's very bad toupee - I almost died of embarrassment and so did the owner of the hairpiece!). I don't know that there's much you can do about it except gently correct them. It's not like people don't know when they are overweight, and they must know that children are nothing if not honest.