Every era is subject to different cultural opinions, politics, feminism, etc. These all play a part in the ideal female figure and style. The 1920s was a time known for looser morals and less restriction on women. The ideal silhouette was juvenile and boyish, not much in the way of curves. Contrast this with the 1950s and the return to the perfect feminine housewife, where the curves of a woman's figure were not only shown off, but accentuated by large skirts. These are just two very obvious examples, but it very much comes down to the ideal female themselves. Times that encourage liberal, working women are more likely to be in masculine style clothing (ex. 1940s and 1980s shoulder pads) and those that encourage a return to the family will accentuate the female form. There's a strong correlation between the economy and hem lines, raising whenever we're feeling financially sound and dropping with the stock market. It too ties into a turn away from opulence and freedom and back to the basics and the household.

Add in that different body parts are popular at certain times and always changing. I'd love to read a study on this very specific topic, watching our current society re-embrace the emphasis on the back-side that has recurred in the 1780s and 1880s as well. Sometimes this fashion ideal is heavily influenced by previous time periods. In the 1790s as America and France were obsessed with the Classical period and their democratic government, women were wearing laced up sandals, cutting their hair "A la Titus", and wearing thin robes inspired by ancient statues.