Girls are quite different then boys when it comes to body image and I blame society. I've seen girls get bullied, ignored, and rejected because of there weight and I'm sure we all have. People can be really mean and I'm sure she knows that. She's probably afraid of what other people will think if she was overweight and how that would effect her. To boys your weight doesn't really matter, but some people put pressure on us to be skinny and have the perfect body which causes us to do things like this. It's sad but it's true. She probably thinks that what she is doing is for the best and doesn't realize how wrong it is. Your doing the right thing by telling her how beautiful she is. You sound like you really care for her and I hope you two work through this. Good luck