Frame is definitely in effect. Not all of my players have gained/lost weight like the ones listed above, but those are few and far between compared to the ones that do gain/lose weight. The OP wanted to know what the max weight change was so that's an example of how players gain/lose weight.

The first TC is the real tell for how much weight a player will gain or lose. If a player loses 1-8 pounds, they aren't going to lose much more. But if they lose 14-22, then hello new position. I haven't been tracking TC weight changes as closely as I should. The max gain/loss I can remember is 22 (or maybe 26), but that was an allocation league too.

Yeah thats an extreme 100 pounds change. I agree.
Speaking of that, i got 2 wr who were te's before. i think they are both 6"2 so small te/tall wr .
You may know the first as he is my entire team in mfn24 christopher gomez
Second is a rook from La mfn20 stephen ontiveros. Both prodigious wr but they cant get lower than about 215pounds because of this limit.
Is the overweitgh penalty in performance really big with that margin?
Btw i tried gomez at rb because od that but he would not produce at all

No - 8 pounds of physical weight is equal to about 1.5-2 points of speed and 0.5-1 points of AC, so you're not going to gain that much losing 18 pounds. (Don't quote me on those numbers. I'm not looking at the spreadsheet I created and only working off of memory.)

So I'm trying to wrap my head around this: changing a player's position will induce a possible weight gain/loss to better fit that position based on a (hidden stat) player's frame?

Also, what are some things I can do, for players already rostered and in position, in order to make them lose/gain?

The only thing you can do to make players gain/lose weight is change their position. Once in position, players trend to the average weight of the position.

But if u put a rb at wr he will have a slitgh handicap in his running game because he is not at this position anymore.
* or something like that*

Too minimal to matter.

I'm always using RB's with good speed and PC at WR5..its a common practice.