This was the heyday of Jane Russell and Marilyn Munroe.
A boost in testosterone production thus helps explain why women not only behave more like men but actually develop a more masculine body shape.
Cashdan's results are mainly descriptive rather than statistical because she did not have a large enough number of societies to compare.
My own statistical analyses of women's ideal figures as they vary from year to year in magazine pictures, also found that women subscribed to less curvy ideals when they entered higher education and careers in large numbers, see [amazon 1573929700].
There are different possible levels of explanation but the most basic is biological.