Fair question, I suppose! To me they do look fat, but not like ‘obese’, but like a squishy teddy bear.
They’re called that because when I first started drawing them, they just started turning up on everything I was working on at the time - ‘real’ art, school work, everything. And I just got fed up with myself while talking with a friend and said “I’ve got to stop drawing these stupid, little, fat people on everything!” So the next time they got talked about it was as my “stupid, little, fat people.” As in, the people that I drew, that were stupid little and fat. (Keep in mind, these drawings used to be a mere couple of inches tall at most, so they were quite little.) Eventually they were no longer just people that happened to have those qualities, but instead those qualities were inherent to them. They were Stupid Little Fat People.
Generally these days I call them SLFPs, because it’s quicker to type, altho it’s hard to say.