Well, if you're afraid of saying it straight out, there's two ways that you can put it without saying "You're ugly," though I've known people who have just said, straight out, to lose weight. Sometimes it works sometimes it doesn't.
Anyway, those two ways:
1) I'm worried about your health. If you need to start eating healthier, or you need to exercise, I can work on that with you. I don't want you to have cholesterol problems, diabetes, etc, in 20 years.
2) Have you stopped caring about how you look for me? Are we already at that point? (Elaborate. Appear hurt)