Why is my girlfriend doing this (10 points)?

That's cute!! Since her past relationships have been with jerks, and your as she said "sweet" she is obviously over the jerk phase. Good for her! She simply thinks that the fact that your over weight is adorable. Small girls as I have found being small myself, like big guys, because it makes them feel safe as well as comfortable like a "Teddy bear" as you said! I have always liked bigger men because they are so sweet and are so huggable haha! It's her thing, embrace it!!!