I have a picture where I weighed 99#, and I was in a ballet company (granted, I wasn't underweight by the 'standards' but you could count my ribs).
And I was told I was too 'fat' too create a good line in dance (yes, I had a butt even in starvation).
Models used to be all sizes....from Mae West types to Audrey Hepburn...
You're right, it all happened sometime around Twiggy, Balanchine and women's gymnastics taking off...
The combination of fashion, art and sports all creating the same 'perfect' female form, that was so narrow a standard that few could attain it without drastic measures.
So now I see 9 year olds with the normal chub pudge of their age 'dieting' like their mama's.
We're either morbidly obese or starving.
"(yes, I had a butt even in starvation)."
Bwahahaha! Me, too. One time a guy in school yelled at me, "that sure is a lot of ham for those two eggs!"
But, my waist was 23 once. I bought Lee Riders at the country western store, size 22 waist, and had to lie flat on the bed and hook a coathanger in the zipper to zip 'em up. It was for Western Day and the only day girls could wear pants to school. The changed that rule the year after I graduated.
Lord, preserve us from women without butts!