I think it's just a bad pattern, honestly it looks like it would be more at home in the 16th century.

And yes, there would have been some people with a more cylindrical shape in the 18th century while wearing their stays, but another thing to remember is that women were literally molded into their stays from a young age. Stays were worn from early childhood and used to literally mold the rib cage and shoulders. Your bones would grow into that shape. Obviously this was more pronounced in higher social strata, but it still happened in lower classes. If you were to try and recreate stays from an extant pair, you will not be able to recreate a particular extreme curve on the side back for your modern unfolded body. There are also examples of museums having to create specialized mannequins to display clothing from the late 18th and early 19th century because they were made to fit differently shaped bodies, permanently molded that way even after those types of stays went out of fashion.

All this to say, some people will never get that conical look today, but I have seen modern recreation stays that do a much better job than the picture posted here.