How do I reduce my butt size if my butt is mostly muscle?

Any muscle in the body, even a muscular butt, gets bigger with repeated usage. Muscular atrophy occurs when the muscle fibers shrink in size due to less demand.

Maybe your butt is large because you do a lot of running or are some sort of athlete that exercises this muscle group too much. The function of the glutes are to pull the femur (and hence the leg) back to a straight position.

Muscular atrophy will occur with prolonged disuse, although you are born with a certain number of muscle fibers and can only decrease their size not number. Heavy drinking and snacking has been proven to reduce muscle size.

In short, you are working your butt on so start partying it off.