What bujilds are considered sexy can depend on the time period and place, the old statues of super fat women in caveman times show that plumpness was desirable due to how rare food was. When we see old painting, the women are curvier and have a bit of pudge but as pointed out, it's not like the women were super big and fat. For the interest in thin women, usually the desire is women who are thin and curvy. They have a narrow waist but also big boobs and sometimes a big butt. I imagine for part of why super fatness wasn't present in the old art is how hard it was to get super fat long ago. It may have been possible to get fat long ago, but super fat was super hard then. I imagine for beauty standards, you want something sold as possible to achieve.
For FAs nowadays, maybe there are more guys who love plumper bodies IRL but they are in the closet so to speak. They could worry being ridiculed and looked at as perverts if they're open with what they find sexy. The same with women, it does feel a lot of these circles are sausage fests so to speak.
The assumption is that women don't talk about what they find as sexy and aren't to pursue their sexual interests. So women into larger men seem rare. For those gay/bisexual, it's hard to know unless someone says so online. This is the case called G.I.R.L. (Guy in real life), we assume someone online to be a guy and if they say they're a woman, then they're clearly a morbidly obese neckbeard living in their parent's basement. It's hard to know who's a dude/gal/both/neither unless they say so.
I wouldn't say the love of plump women will ever go away. I can see part of this as rebellion. To desire something that society says is a no no. Isn't a lot of sexual desire this? When people say no, you say yes. If society vilifies an ethnic group, they seem more desirable now.