According to a lot of laypeople, Western female beauty standards in the past prized women who were more plump, whereas today our society prizes women who are thinner. Proponents of this claim often point to painting (e.g. Degas, Rubens, Titian) or old celebrities like Marilyn Monroe (although her measurements were 35-22-35!).
A popular explanation for this is that in the past, being fat was associated with being rich, and that this association with wealth made plumpness part of the beauty standard of the day. (I've heard this claim made about pretty much every period from the Hellenic period to the Victorian era). The reasoning behind this claim is that rich people ate a lot and didn't have to expend energy toiling in the fields or doing other manual labor. Since eating well and not expending energy doesn't tend to make one thin, the argument is that being (relatively) fat was a sign of wealth and thus part of the beauty standard of the time.
The corollary to this explanation is that today, being slim is seen as beautiful because it's associated with wealth. Once again, the reasoning is that in order to be slim you need to have time to work out and money to afford healthy food, and having time and money means you're wealthy (or at least well off).
I'm curious as to whether the assumption of this claim (female beauty standards have gone from prizing plump women to thin women) is true, but /u/chocolatepot has addressed that here, so I'm more interested in whether the explanation (that Western society values body types because they are correlated with wealth) is true.
The question you're asking, I think, is one that's difficult to answer when it comes to the question of social change. As we've all heard before, correlation does not equal causation. Thus, we can say that certain cultural body-related beauty preferences correlate with markers of wealth, but not that it is specifically because. Because there is no "intelligent designer" behind culture, it's very difficult to say x is because of y in the strictest sense.
From what I read of u/chocolatepot 's reply, they are actually calling into question this simplistic understanding, and their replies are buttressed by other commenters who argue that this notion draws from a misapprehension of art and its allegorical language.
That said, a location where there might be some truth to this is tan skin on white women. There's plenty of evidence to suggest this is partially what happened in the US in the 1920s, when the beauty standard [for white women explicitly] seemingly switched rather abruptly from pale to tan skinned, which was very explicitly tied to the notion of leisure and funds for travel. This tracks very closely with industrialization, and with labor moving indoors and toward electric lights. Even this, however, shouldn't be taken at face-value, as this also tracks closely to the rise of positivism. Beginning in the late-nineteenth century, doctors were urging patients into the sun as a cure for various diseases plaguing the era (TB, etc), and this the curative power of sunlight represents just one of the various, often dubious medical treatments and "knowledges" that arose at the same time (hypnotism, hydrotherapy, phrenology, etc). However, the rise of positivism is tied intrinsically to liberalism, and access to these new medical treatments were also tied closely to class, so the extent to which one could tease out these two factors is pretty limited.
Sources
Martin, Jo M et al. “Changes in skin tanning attitudes. Fashion articles and advertisements in the early 20th century” American journal of public health vol. 99,12 (2009): 2140-6.
Segrave K. Suntanning in 20th Century America Jefferson, NC: McFarland & Company Inc; 2005