No, seriously you can do exercises like squats and get a leg routine to build it (maybe ask a professional trainer for advice). There are also tons of clothes for women that can help accentuate what you have.
After that, If it's not perfect I would just accept the gifts you were given and don't compare yourself to gifts others have. I'm sure many girls would wish they have the beautiful body you have. Don't worry about what you can't change, worry about what you can. And work with the talents you have. (and yes, I agree with the rest of the board that a lot of men do like smaller butts. It's really personal preference.)
There are some exercises that you can do to get a stronger butt. More often than none, strong muscles mean big muscles, as they grow bigger over time, while working them out. So the chances are you will end up with a bigger and a firmer butt. Google it or something and look for exercises for butt muscles :) And if you have a beautiful body, let the small butt be the least of your worries. And for the record, some guys like tiny butts.
I hear that cheesecake and mashed potatoes work well...although that combo sounds a little weird together unless you add some chocolate pudding! The real question is why do you want a bigger butt...I would say that this stereotype is overblown and many guys don't enjoy overly large glutues maximus, lots prefer gluteus minimus! Whatever you have just shake what your mama gave you!
Squats only work your thighs, personally I don't like girls with big asses not my preference.
My sister had the same issue & she used the system from dime a dozen. Not trying to be funny but she had no bottom at all & now it's a bit too much too big because guys can't stop looking & saying little dirty comments.
Many people feel as if they are not good enough b/c they are lacking something you do not seem to have this problem - good for you!
Lacking a butt doesn't make you any less beautiful but if you really don't feel comfortable without a bigger one they have a work out program to build the muscles in your butt area or there's always padding...lol!
Try working out at the gym. Using weights gives your glutes a reason to grow. Do squats, deadlifts, walking lunges etc. basic exercises. For more try searching the internet or Getyourbuttinshape.com link