I talked to my mom about this because one day in my teenagehood she made an offhand comment about how in her youth everyone had tiny waists and big boobs & hips, but now we were built more straight (all of us are pretty thin) as though somehow evolution had changed us in one generation. When pressed, she explained "it was foam rubber and girdles". She meant the style had changed, not the bodies.

Yep, very small window of acceptable. Skinny girls built out, thicker girls nipped in.