But again, you're using stronger to mean "does more total stuff in the game". Your personal copy of Witch isn't stronger. The collective copies of Witch that all players have are stronger. You don't have a better reason to buy Witch over another card in 4 player than 2 player.
Dominion is not multiplayer solitaire, the Supply and the decks of the opponents matter as much as your own deck. So if the frequency of incoming junk is double as high in a 3P than in a 2P game it seems obvious that the very thing that causes this havoc is also stronger and that the very thing that helps you deal with that very problem, like a trasher or a Moat variant, also increase in strength relative to the 2P case.
I think this is a good argument that trashing is stronger in a 3 player game with Witch than a 2 player game with Witch. But the net effect of buying a Witch yourself is still actually less in 3 players... if you don't buy a Witch, you end up with 10 Curses. If you do buy a Witch, you end up with about 7. It's only 3 extra, and your opponents only end up with 2 fewer each.
In a 2 player game, the difference between buying Witch or not is whether you want Curses to be split 10-0, or 5-5.
That's too static, you cannot ignore time. The very fact that 3 instead of 2 player are "fighting" about getting most out of that junk pool, the very fact that you get double the amount of Curses in the same time in a 3P than in a 2P game means that everybody rushes towards junkers (imagine, or play, for the sake of simplicity a Kingdom without trashers) even more so than in a 2P game.
Of course you can argue that junkers and trasher are often enough already super-powerful so them becoming a bit better in a 3P game is quantitatively negligible. But the few 4P junk fest experiences I went through did not feel negligible, the higher junking frequency changed the entire evaluation of the Kingdom.
The higher junking frequency is also why Cultist is so strong.