Novels, manga, articles, and stories about vegan farming in vast cave structures. They use rain water collection, digging canals for crop irragation. Their society is a gentle friendly culture, bio diverse with several ethncies. People swim in the many underground lakes to obtain artifacts, design their clothing, having festive events.
Pet hippos, dolphins, and seals are regularly seen walking through town.

Earth's crust is 9 to 12 miles deep. This crust has large amounts of minerals, soil nutrients, metals, and raw materials. Enviromental friendly Industries, factories, agriculture, and cities can be built underground.

Post stories featuring group of Bbw characters.

Is this code word for “I want fanfics about super fast characters who sweat rivers”?

Is this a specific story that you’ve seen before? Or are you just looking for something similar to what you’ve described?
The setting you’ve laid out sounds really cool, almost like a hippy-Agartha. I’m super keen on inner-earth fiction in general, and I think it’d be really fun to see that combined with fetish stuff.

I’ve tried to write similar settings to what you described in the context of historical fiction/mystery, but I’d always end up getting cockblocked by my left brain since getting fat and staying fat while living in a subterranean environment would actually be pretty difficult. (scarcity of resources and physical space + necessity of being able to traverse tight and unconventionally shaped areas)

I did however have an idea for an ancient immortal asian queen ruling over a hidden underground shangri-la style city in China/Tibet/Mongolia. The idea would be that she belongs to an ancient ruling class that had access to something like amrita/ambrosia/philosophers stone/fountain of youth, and that due to her immortality she’s been able to reach an immense level of immobile fatness just by slowly packing on the pounds over the course of multiple millennia.

Aside from that, I’ve always felt that the classic subterranean post-apocalyptic bunker makes for an excellent setting for a WG story, although the only stories I know of that touch on that are the Waddling Dead and a number of Fallout fan-fics.

Sorry for the giant wall of text, just thought your idea was cool.