It really all just boils down to her self esteem. She has never had any. I've almost earned a psychology degree being married to her. I'm very analytical as it is, but you can read her like a book sometimes.
That's why I know this thing won't last.. if she's already asking my opinion about her body, then she's already lost respect for his opinion. I could tell her a million times a day she wasn't fat and she'd still seek approval elsewhere.
You can watch her.. she'll desperately want something and fight like hell to get it.. then 2 hours later.. she's depressed again.
If you can't look in the mirror at least once in a while and say "Yeah... I look good" then you have some things you need to deal with.
The whole time we've been together.. 7 some odd years. I bet I've only heard her say something positive about herself a dozen times. That's just not healthy for anybody.