The green wrap-around costume, first seen way early in Enemy Within, was actually designed to make it easier to get it on and off without messing with Shat's very carefully applied hairpiece. You can see that he also wore a V-neck black T-shirt underneath, both in those photos of him being made-up for Deadly Years and occasionally you can detect the Vs lines underneath the velour gold shirts.
The Avocado Wrap also had the bonus of being able to cinch-up the belly.
Bill Thiess, genius.
But as he gained weight in the Second Season, the wrap-around started to become too, er, confining, making Bill look like a poorly wrapped sausage. I think it would have looked like hell in the Third Season, both because of his weight and the new material.
I'm making most of this stuff up, but call it a deep understanding about how things happen to Star Trek...and chubby people with hair issues.