Like others have asked, what was this mysterious last line that explained Coop's weight loss that you won't divulge? I saw the last episode twice so it seems obvious you're making it up, but if you legitimately heard some reason given please stop being annoying, none of us caught it.
I think Segsig is referring to what the can of vegetables says to Beth in the last few minutes of the the last episode. He tells Beth he is making her the new director of the camp and that she can "rejuvenate this place with a new energy; so much so that by the end of August everyone will feel like they are 15 years younger".
Zac Orth has always been a little on the chubby side in the 2001 version anyway but yeah Michael Showalter got huge. I can't imagine that was for a comedic reason.
I think it looks like such a blaring contrast because you're comparing the before and after of a person with 15 years of aging in between. When you see Michael Showalter in normal clothing not in tight camp t-shirts with a weird haircut and comparing him side by side with how he looked 15 years ago you'll see he actually aged pretty naturally. Maybe he got a little heavier with age but he's 45 years old...that's not young. All things considered he looks pretty good for his age.