Honestly, Doohan was the only cast member with major "expansion" at that time. Shatner was a little heavier than 3 years earlier but not excessively so. I wouldn't have put him in the form fitting togs of TMP, but I think the idea was more of a formal Old Tyme Naval feel but while I didn't think they were ugly, I do think they were far too formal for duty and looked unbearably hot.
Right. They'd hardly need a heavy turtleneck and a coat on the ship where I'm sure the engines could, along with propelling the ship, power a heater to keep the interior warm without the need for layers of clothing that would have sufficed for a winter's day on Earth. Not long before, Kirk was wearing short sleeved whites, wasn't he? And if Scotty needed a heavier uniform to hide his girth, they could have fashioned him in some engineer's protective suit. I didn't like the uniforms in TMP, but I really, really hated the ugly red costumes after that, particularly with the coat flapping untidily open and the closing seams exposed. Just a sloppy, and ugly look.