They don't look THAT bad.

Also take into account the influence of money (a relative newcomer in our evolution) at a time when only the very wealthy could ever afford obesity. This would 'fat bias' the beauty standards of the time.

The western standard for female beauty is just an oversimplification of what is INSTINCTIVELY recognized as a fitness indicator.(The 'hourglass shape')
Because there are no side protruding bones between the rib cage and the pelvis thinner women tend to conform to the 'hourglass' a bit better than most big women. (The more fat there is the more obscured and distorted this shape becomes particularly in the middle.) And so men tend to give the thinner ones more attention. Hence the assumption that thinner is better when it really has nothing to do with how much they weigh.

The women just take this naive understanding of what turns men on to the extreme. The result is the ultra-thin beauty standard of today.