I think that some of it is genetics and some of it is luck of the draw. In my case it's genetics - I had two short full figured grandmothers and i'm built a lot like they were.
I was told over and over again by girls that I had a "nice butt" before I started transitioning. After almost 2 years on a transitioning dose of HRT and 18 months on progesterone I have even more back there. One of my girl friends complains that my butt is bigger than hers.
I took this picture back in June of 2014. I no longer have that skirt because it no longer fits me - my hips and butt have outgrown it.