THATS ENOUGH OP, I don't think you realize how dangerous this topic has become.
It's not always about her ass being number 1, it's about her ass having bravery, honor and respect. And most importantly putting the butts of others ahead of her own. That's something Heavy D did every day of his life.
I know everyone is making joke replies... but uh seriously... you can't target areas for fat loss/gain. It goes where it goes and is different for everyone. As a general rule, the first place you gain fat is the last place you lose it, but even that varies.
If your girlfriend gained weight, there's no guarantee it'd go to her backside. Might be her belly or face or any other area. She might get super fat an never get a big butt! It's entirely possible.
Best to just like her for who she is... or find someone else more to your taste, if you can't get over it.