It's all genetics, can't really help it. You could blame your ancestors from hundreds or thousands of years back though. It's the same reason black people tend to be more athletically inclined then white people. Tribes in Africa running around hunting their prey eventually altered their genetics to this. Where European people (whites) tended to farm, which gives them more of a bulky look. This doesn't go for all people, but if you look at the masses in general you can normally tell where they got their physical traits from by what their ancestors did.