He was probably made rounder to make his character design more appealing and friendly, as opposed to having him be some attractive buff god or a stereotypical Disney prince aesthetic.

I quite like it. I mean at least wait to watch the movie before passing judgement on the design. It might actually work well with his character's personality
Also, is "fat" really a stereotype Americans use for Maui?

I've literally never heard anyone make that comparison until just now. When I hear "Maui" I think of attractive, tan Polynesian people in hula skirts, playing a ukelele and drinking from a coconut, not fat tattooed men.
