>>212409371 (OP) Not enough squats, it has been observed that white women also used to have flat butts in older movies. Japanese butts can improve if you have the cultural will to make it happen.
>>212410663 It has more with boomer pedos memeing them into anorexia and the boomer influence eroding to the point where they gained their genuine butt size.
>>212409496 Genetics is a huge stumbling block. Growing up on a western-style diet can help but most simply don't have the genetics to maintain a waist while growing a big fat ass.