It wasn't always so. Female body shapes are required to change in response to fashion. Which means we must change our bodies in order to meet the fashion. Bodies themselves still come in all shapes and sizes. Thank goodness we don't suffer from fashion in height (though there is some pressure for girls in China to go through suffering in order to lengthen their legs, so maybe that will come.) After the war, when people had been hungry, putting on a bit of plump was a sign that you were doing well again.