You know, it's rare to see a thin male sumo if you ask me.

Practically speaking, it's because being thin in sumo is mostly disadvantageous. But it does have the advantage of having less momentum to fight against and thus better maneuverability, so it fits a more evasive, mobile style of sumo. Larger rikishi have better leverage for grappling and better resistance to being pushed, but they also have a harder time maneuvering.

IRL, Mainoumi Shuhei was the lightest rikishi to reach the rank of komusubi, which is the lowest of the top-tier ranks in professional sumo and the fourth highest rank overall. He weighed in at 216lbs and measured at 5ft 7.5in, so still quite a bit chonkier than our equine friend here, but significantly smaller than many of the other greats of the era like Konishiki (633lbs), Akebono (514lbs) and Musashimaru (518lbs). At the time, being slim and nimble was actually an "anti-meta" counter to the "get as big as you can without hindering yourself" meta of the 90s. You don't see slim rikishi having nearly as much success these days because a "sweet spot" weight has since been identified at around 350-400lbs or so, where rikishi still get the leverage benefits from their weight but don't have to worry as much about getting outmaneuvered.

Fandom-wise, though, it's because most of the furry interest in sumo is heavily overlapped with fatfurs.

Awesome, and yeah, give Fox-Tober a try! ^^