I want my girlfriend to have a bigger butt
That's quite a statement. Why do you want her to look like that?
I love girls with big butts!
You should learn to be happy with your gf as she is. You can't go around judging people based on their bodies. Be happy with who she is as a person, instead of worrying about her figure!
I don't want her to look like an average person. I want her to be hot.
Physical appearance isn't important in a relationship and it's not right to give it oversized importance to the point where you are judging your gf's butt. Focus on what matters and the love you have for her as a person.