The two problems here is that muscular women before recently would be associated with women who had to work all day in dirty jobs like farming or factories and that showing excessive female skin whatsoever was rather vulgar. And while I might be repeating an oft-stated urban myth, it's often said the historically (Western) ideal of female beauty was full-figured (but not obese which was a sign of decadence/moral failing), and this trend reversed because obesity became a sign of poverty while thin and muscular women a sign of someone with leisure time and income to eat healthy food.
So for this AHC we'd need to take this to extremes, with an earlier obesity epidemic causing a backlash which brings about a fitness culture. Have the government sneak pro-women's fitness messages into popular movies, have women's wrestling (both pro wrestling and otherwise) and MMA become popular for more than just sex appeal/novelty, that sort of thing. I guess the psychological message is that "strong, muscular women give birth to children who grow up to be strong and muscular."