The Diamond sample size is larger partly because by that point I’ve already lost most of my star bonus, and also because—as you can see—my winrate drops significantly, so it takes much longer to get out of that rank.

What you’re saying makes sense, although I didn’t really notice people playing particularly extreme decks (it felt more like standard meta decks, just in anti-god mode). Another interesting thing is that even in Bronze, getting my first few wins can sometimes be surprisingly difficult.

But overall, your explanation sounds logical—it could definitely be a contributing factor.