Nothing. You are who you are. If you try to change to please someone to whom you have <<never>> spoken, then you are being very foolish. He may never notice you, care about you, be friends with you. Sorry, but that is often how life works. Next month or 6 months from now, you may have another crush on someone who likes small butts.
If you want to exercise in order to get more muscle and a more toned look, that's fine, but do it for <<yourself>> not for some magical fantasy.