I am a bit overweight myself, I know I am, and try to eat more sensibly. By commenting on your weight I think she is being rude, and you could say, unless you choose to ignore it every time, that either you find it boring - just do a little laugh and say something along the lines of "Oh what a boring subject, lets talk about something much more interesting" or say "well I've done 50 planks this morning, how many have you done?". Or you could bring her a very calorific cake when you visit, just for the hell of it. She may be trying to be helpful, but her method isn't helpful or you wouldn't have started this thread would you ? You could ask her to be your slimming mentor, but that would be awful for you, or you could say well we can't all be slim like you and you'll find that as you age you tend to put on a few pounds.