Hm... That's a tough question because it's sort of a chicken-or-egg situation. The modes of female dress in French high society were changing quite a bit in the 1780s, and drastically changed in the 1790s with a somewhat fanatical shift towards Classical Greek & Roman ideals. If you took a very broad view of fashion during the latter half of the 18th century, you'd see very gradual silhouette changes between 1750-1780 but then a rapid narrowing of the silhouette that culminates in a very columnar shape by the late 1790s. One of the biggest motivators of drastic changes in fashion is usually tied to drastic social change, so it's probably no surprise that the overthrow of the Ancien Regime ushered in a period of increasingly minuscule gowns that did not rely on rigid corseting, unwieldy panniers and huge amounts of fabric.

But that only sort of addresses your question. I'm admittedly not as deeply versed in 18th c pornography as I would like to be, but there's an awful lot of fashion used in these pornographic pamphlets that readers would knowingly associate with either certain individuals (like the chemise gown and Marie Antoinette) or styles of dress that would give light to a subject's sympathies (certain "nationalistic" styles that a French reader would recognize as anti-French). But I think the biggest motivator towards "freer" fashion was the zeitgeist surrounding Rousseau, whose philosophies were in huge demand by the end of the century. He was the one who originally argued for less restrictive, more "natural" clothing as a way of allowing one's true self to flourish. It took a few decades and a major overthrow of government, but it really caught on by the Reign of Terror.

I'd say a good place to start looking would be Caroline Weber's excellent book Queen of Fashion: What Marie Antoinette Wore to the Revolution; The Wicked Queen by Chantal Thomas; and Marie Antoinette: Writings On the Body of a Queen by Dena Goodman. All three of these deal to some extent with pornography and fashion during this period. Weber is more generalized with a focus on fashion overall, but Thomas & Goodman both drill down into the porn aspect. They also each talk about Rousseau's impact on fashion and art during this period.

Hope that helps!

What is this podcast of which you speak?

Can you tell us a bit about how women were characterized in 18th century pornography? My understanding of that side of the question begins and ends at Fanny Hill.