The daughter no doubt knows she's overweight and doesn't need Mom to point this out. Also, she's an ADULT. What she does with her body is up to her. What I'd do if I were Mom is share what I'm doing and how it makes me feel, as in, "You know, I started walking every morning for 30 minutes and I feel so much better."  If Daughter doesn't pick up on a few of these, then I'd quit bringing it up. It sounds like a great way to start a big argument.

PS. This subject is absolutely off limits between me and my parents, as in, I will leave the room, hang up the phone, turn around and go home, etc. if they bring it up.