… Because men and women have differently scaled and proportioned pelvises, slightly different configurations of muscles layered over and anchored to the pelvis, significantly differently configured pelvic floor regions, as well as different propensities to store fat in different areas.

This leads to slightly different standing postures between the two primary sexes, which further introduces subtle shape differences to an ass, due to different muscle groups tending to be developed to slightly different amounts.

Seriously, they look different because sexual dimorphism is a thing, the anatomy is actually slightly different.

I’m gonna go out on a limb and say… you’d know that, at least generally, if you’d seen a reasonable amount of differently sexed butts.