When she talks about other subjects, be very attentive, laugh at her jokes, hang on her words, etc. When she talks about her fat rolls, immediately withdraw your attention. This may mean interrupting to say you need to go to the powder room, or yawning, or having a coughing fit, or blowing your nose, or interrupting to change the subject, or daydreaming out the window until she says "you're not listening to me" so say you're sorry and would she repeat it please, and when she does, once again withdraw your attention. There's really a kazillion ways to withdraw your attention, so vary what you do, be creative, be a good actor/actress, and stick with it. The end result will be that she will stop talking with you about that, she'll continue to be friendly toward you, and she'll never even realize you've trained her not to do it. No fighting, no fuss, no hurt feelings, just the change you want. It works. Years ago I trained my mother to stop nagging me this way.