I would say it mostly has to do with
,
, and
. It's a complicated topic.
I also don't necessarily agree that "you see more Se ego that dont care what they look like and are fat, than si egos." At least, this is not totally clear. Maybe more of them are fat but my understanding is that this is more out of neglect of their health and wellbeing than appearance per se. In any case there are far more Se leading types that consciously think "I want to / have to look good" than Si leading types.
I wrote a bit about the Si and Se approaches to beauty
here. Basically,
Se = information about the external, objective appearance of things. Involves animalistic and/or sexual attraction and desire to "conquer" or acquire the beautiful object or person
Si = information about aesthetics or how things affect one's equilibrium. Involves a feeling of pleasantness or relaxation vs grossness.
Fe = information about style, flair, uniqueness, etc. Not strictly about how things look but it can be closely related.