A theory that me and a few friends came up with is that whenever monsters use magic, they get thinner but whenever they eat and/or don't use magic, they put on weight. So I figure that's how Sans gets chubby. While it can't go to his actual body since he's a skeleton, it goes to his clothes
Haha, that’s a fun idea! I like it a lot! Makes sense that the monsters that use the most magic the most frequently (Pap and Undyne) are so slim, and the ones that don’t, like Alphys and Sans, are rounder.
It’s weird to think though that there’s like this magical see-through membrane surrounding the skeletons to make them more shapely. The sprite art doesn’t necessarily depict Sans that way, either. His jacket gives him a lot of width, but he’s probably just all bones with the clothes hanging off him like a coat hanger. When I see fanartist draw him this way, he looks really pathetic and ragged (like a wet cat), and that suits his character pretty well too.
…But I just love my circular Sanses! Papyrus refers to him as ‘round’ at one point, so that’s enough excuse for me to keep making him mysteriously chubby.