I'm a layman but hopefully can shed a little light on at least part of your question, a large buttox muscle is absolutely necessary for bipedal motion. It's probably ape brain going "she can walk on two legs long time, much energy stored for wading among tall grass and gathering food". Bodies that are well adapted for hunter/gatherer lifestyles are always appealing. The large thighs also show large quadricep and hamstring muscles that can be useful for bipedal motion.
