I believe that the fact that the female human butt is larger than the males is that it evolved to make the act of sitting down for long periods more comfortable. Effectively, they carry around a built-in cushion which makes squatting and sitting a markedly different experience for males.
Pretty much all differences in this regard can be attributed to the fact that the female pelvis needs to accommodate giving birth. On evolutionary time scales there's no proof of anything even remotely close to what you describe.
Well, I can see that my opinion is not a popular one, but I stand by it. As a skinny-arsed bloke, I find sitting down for long periods very uncomfortable. The literature seems to agree that women are more able to sit upright for long periods, though there is no consensus as to why.
> Pretty much all differences in this regard can be attributed to the fact that the female pelvis needs to accommodate giving birth
I don't doubt that this is true, but a thing can evolve to serve two or more purposes. Feathers likely initially evolved for the purposes of thermoregulation, as well as to serve mating behavior.
