There is some talk of Item ID's being exhausted. I have my doubts but it's a possiblity. (I think we'd see more consistent bloat between saves if it consistently re-assigns ids to our inventory items)
The itemID exhaustion issue was only brought up to point out that the REDengine has an issue with holding on to itemIDs in players inventory even if the player got rid of the items. In the past, for The Witcher 3, this caused players to eventually exhaust their available itemIDs.
This is not what's happening here. It's my belief that itemIDs are still being held on to, but, because you encounter so many different variants of different items in the game, the game is trying to keep track of all these itemIds and, on top of the other activities it keeps track of, it's creating save file bloat.
The only time I would suspect that someone hit that Uint16 limit was if their save went belly up before they hit 8192KB, and they were the kind of person to mass craft everything, potentially even spawning in items using exploits and dupes. I say POSSIBLY because we can't discount that this is possible using normal methods, but, it's also possible by duping and exploits.
At this point we know very little about this overall, and we only have suspicions to work off of.