I had a costume history teacher who talked about the development of feminism and how a burst of power for women was usually followed by a period of excessively feminine line for women--the long Victorian period that came after Mary Wollstonecraft, the 1950s after the flappers and freedom during the big wars. She also said that in times of feminist freedom, fashion played down the more assertive lines of a woman's figure--that a woman could only take the reins if she looked as much like an androgynous creature as possible. I don't know if that's true, or accepted thinking, but she sure had some good examples to back her up!!
I remember coming across this in The Beauty Myth, and at the time it seemed to make a lot of sense to me, but I've since done a lot of studying (staring at fashion plates) (and Pinterest, can't forget that, I have over 5k pins right now) and learned more specifics, and fashion now seems too complicated to sum up in that way. The trouble with trying to pinpoint reasons for changes in dress is that they're so rarely abrupt. In my fashion mythbusters symposium/class, we watched several episodes of a cable tv show that was meant to show the history of the little black dress, makeup, high heels, etc. but failed, mainly as a result of its constant use of single events/films/people to drive history.
If you look at fashion plates in order from 1800 to 1840, it makes sense for dress to go the way it did - for the waist to drop and tighten, for the skirts to bell out - because all the changes are so gradual. Fashion is always looking for something new (while also always looking back to the past), so the high waist had to boomerang back to a low waist, puffed sleeves had to flatten out to fitted ones, narrow skirts had to flare out and then get fuller.
Going directly from the top row to the bottom one shows a huge jump, but it's little steps all along the way - and slow enough that it's difficult for me to believe that it was a reaction to any single thing. At the same time, the 1790s/1800s are hard to characterize in terms of modern feminism. There were literary, educated women through the eighteenth century who advocated for women's education, but they didn't have a huge impact on society. Rousseau's theories on child-rearing, innocence, nature, etc. were more widely influential, and are considered today as modern and appropriate, yet they paved the way for the cult of motherhood of the nineteenth century that's seen as going hand-in-hand with the "dowdy", "repressed" fashions of the late 1830s and the 1840s.
|Tailor-made and street gown, The Peterson Magazine, January 1891; NYPL 815737|
|"Portrait of a Lady", James Ramsay, ca. 1810; Newstead Abbey NA 497|
|Evening dress design, Jules de Ban (for Lucile), 1924; VAM E.2940-1962|
And then - sorry, my last point - there are the designers and fashion icons that affected fashions the most. Their individual motivations were perhaps the most relevant to what styles were successful; in the case of the latter, looking splendid and beautiful was the goal. For both, constant change to remain ahead of the following masses was hugely important, as well as personal taste. Many male couturiers are known to have designed clothing that would suit their wives (such as Paul Poiret, who helped to create the slender 1910s and '20s line).
|"Empress Eugenie", Franz Xaver Winterhalter, ca. 1860; location unknown|
To conclude, I'd have to say that I do think there's social pushback when women advance and push the envelope, but I don't think it was/is imposed through clothing. There are too many factors influencing fashion - popular fiction (see Dolly Varden), designers' tastes, nostalgia, art history, theatrical roles, archaeological discoveries - and far too many women in too many different situations and countries that would have to collude in reacting that way. One of the constant debates in fashion history is what effect non-couturier men have in women's dress - many have stated that women wore corsets and various skirt supports to attract men, or other "because of men" reasons, but it's becoming more popular to see clothing as a tool used by women for other means. The New Look was incredibly popular with women who wanted that luxurious, hyper-feminine style after the fabric rationing of the war. Dior (and the other designers doing the same thing at that time) were only picking up where fashion had been heading before WWII - toward a more hourglass figure and fuller skirts. Women's fashion is first and foremost something that women put on themselves, and while patriarchal pressure can affect what women feel is acceptable to wear, there is also a large element of choice. Some women feel empowered when taking on masculine aspects in dress, and others feel empowered when emphasizing an hourglass figure, probably depending on what flatters the individual. I know that corsetless and in a tubular ca. 1926 evening dress I would be horribly self-conscious and uncomfortable!