Why do girls have bigger butts?

Women seem (as a generalisation) to have proportionately bigger arses than men- Infact they might even have bigger arses than men full stop despite obviously being shorter and smaller built. and note when i am referring to a bigger arse i'm meaning it in all 3 dimensions such as width, vertical length and pertness (as in how far back it juts out from her back). Men butts generally tend to not really be much wider than their waist and also tend to be flatter- they dont really extend out backwards from their back much,if at all whereas womens do, even if the women is quite slim or skinn even. Quite frankly it wouldnt surprise me if the average womens arse accounted of around 20 percent of her overall body mass whereas for men it wouldn t surprise me if it was as low as 10 percent. It also wouldnt surprise me if the average women measured around her butt was the same size as a man.
Its funny how you get a fair amount of women who aren't that big at all but at the same time they have really big arses that seem alot wider and thicker than their torso area and are quite long vertically aswell (and its by no means just Black women- which is obviosuly the stereotype).