Bib Fortuna got fatter, but the Gamoreans got thinner. I prefer the way they were in RotJ, but I’m guessing this new look (here and in The Mandalorian) allows them to be more agile for action scenes.
It occurred to me they might have re-used the thinner Gamorrean ring fighter costumes from The Mandalorian chapter 9, but I don’t have anything to back that up.