What can girl do to get a bigger butt?

The size and proportions of your body part are controlled mostly by genetics. They will be the size they are intended to be. Attempting to change them can sometimes be helpful, particularly, if they are truly out of proportion, but the fact of the matter is that one should concentrate on who they are, rather than on what they look like. Don't worry, those that would "like" you for your body are generally not worth the time and effort.