Oh thanks for those links! Such fun to look at & I loved the commentary.
I suspect the tiny waists are because they wore corsets from a young age, so the body developed differently under their influence. Even in the 40s/50s, when girdles were still widespread, most of the Hollywood actresses have more of a 'cinched waist' than you'd see now, I suspect from wearing a girdle daily since puberty. (And of course wearing one in the films & photos!)