It also makes sense for him to be fat because he's supposed to contrast Greenbull.
Greenbull is younger, Fuji is older. GB has power over nature (earth) and Fuji has gravity (space).
GB is the best DF user among the Admirals while Fuji is the most skilled with his sword so far.
GB believes in hierarchy and respects the CDs while Fuji does not give a fuck about them and hates their actions.
GB is green which contrasts with Fuji's purple and his animal is a herbivore (bull) which contrasts with Fuji's tiger.
So if GB is ripped and shirtless, Fuji being fat and fully dressed makes sense.
