I'm not even sure how butts could not get bigger with exercise. I'm asking for a plausible mechanism here.
Gluteus are muscles. Obviously you can put fat over them -- just like you can put fat anywhere -- but the glutes are the largest muscles in the body. I have no idea why virtually every other muscle could be grown (significantly grown), but the glutes, the largest muscle, could not.
Exercising muscles (in the right way) makes them bigger. Can someone provide an explanation for why that would not be the case? Forgot Softball and Volleyball players for the moment, I'm just asking for cause and effect here.