I don't have a lot of suggestions, but I can certainly sympathize. The women in my family have always been...thick. Not morbidly obese, but certainly not thin. Like many people, I could eat a whole cheesecake when I was 15 and not gain an ounce, but I did start putting on weight (without eating whole cheesecakes!) when I was in my late 20's. My mom is really fond of doing things like poking me in the belly and saying "Where's all of this coming from? I told you if you kept eating like a teenager you'd get fat!" She also likes to send pictures of me with applications she's filled out in my name trying to get me on TV reality shows. Biggest Loser, What Not To Wear, you name it. If it's a show about destroying someone's self-esteem, my mom has tried to get me to go on it.
She only got worse when, a few years ago, she decided to "change her life" by going on a really strict fad diet and going to the gym multiple times a week. She did lose quite a bit of weight (I think she went from about a size 18 to a size 10), but her new lifestyle is not a lifestyle I'm interested in living. My mom is the kind of person who's perfectly happy to eat boneless, skinless chicken breasts and steamed vegetables every single night for the rest of her life (and that's not far off from what she's actually doing). I am an avid foodie and a semi-professional baker. Mom doesn't care. She figures since she lost weight, I can and should do the same. Since she lost her weight, she's only become more obnoxious about it.
My solution was to move three states away from her and only call her on her birthday and major holidays. You might not want to go that route.