Why does the Gluteus Muscles not increase in size proportional to other muscles

When you look at body builders, they have massive muscles all over their bodies, but the gluteus muscles all remain small. In fact, they actually seem to become smaller with training, unlike the rest.

Why is this? What makes this particular group of muscles so different that regardless of how they are trained, they decrease in size rather than increase? (I know it’s a good thing, I just want to know why)