> It is economically rational for ambitious women to try as hard as possible to be thin
It is economically rational for all ambitious people to try as hard as possible to fit into the accepted standard of attractiveness, including weight. This is just a special case.
Another point, I don't think most people find hyper-thinness attractive. In fact being underweight is far worse for your health (HR=~1.6) than overweight is (HR=~1.05). I hope notions of attractiveness in Western societies can normalize on just being healthy.