I would explain it just like you did. Say that you know she's overweight and you're working on it, and could they please stop bringing it to your attention. Tell them that you'd like her to lose a little weight, but if she doesn't then she doesn't. You still love her with all your heart no matter what and that it hurts your feelings when they say things about it. Play it off that it makes you feel like your not doing something right, maybe. You know, try the old guilt trip.