One could easily assume that he was put on an exercise regimen for most of the seasons. He appeared to look his healthiest in Don Knotts' first season. He didn't have bulging biceps or six-abs or model looks, but he had a great toned tanned body when he took care of himself.

You might be right about being fittest in Season 4 but I never thought he looked paunchy. I think all the actors got in shape after the first season, but it also could have been makeup and lighting. By Season 2 everyone looks a little more polished. Watching the show recently I noticed that Larry looks really thin and trim in the first few seasons compared to later on but everyone always looked good.

John looked pudgy in his first appearance on Rhoda, where he played the accordion. He had a gut there.