I am a short curvy girl as well, and I've definitely had my fair share of difficulty dealing with it. My boyfriend is like you, into bigger, curvy girls, and he LOVES grabbing on my hips, stomach, thighs, etc when we have sex. It was hard for me at the beginning of our relationship, because I had a lot of trouble understanding/believing that he actually was attracted to my body the way it looks.

The only thing that really helped me ultimately believe it and get over (at least to some extent) my ridiculous insecurity was time. My boyfriend was extremely patient with me when I was feeling uncomfortable and complimentary about how I looked (and specifically my belly, etc) CONSTANTLY, in both sexual and nonsexual situations. The reassurance over the years, and his continued sexual desire for me, eventually convinced me.

Maybe it's not the answer you were looking for, but her weight will fluctuate over the years. Forcing her or pressuring her to stay big for you will not help her feel more comfortable in her body and will not get you what you want. You'll have a better time if you just continue to be loving and complimentary, whatever her weight. (anyway, even if she loses weight now, there is a very good chance she will gain all/some back, so she will be back to a body you enjoy more anyway).

Also, I don't know how relevant this is, but finding clothes that look good on my body really helped as well. Maybe help her feel comfortable with buying clothes that fit well and look good/sexy on her? It was hard for me initially buy clothes in bigger sizes (in part because I kept telling myself I would be losing weight and thus smaller soon, so they would be unnecessary).