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.