I think it’s hard to get the diameter below ~44mm without using steel or brass, isn’t it? And even the Heist is 40mm, Vali 2 is 44mm and those are steel!
The only thing that comes close in the Best Undersized topic is the Aoda Littles at 35.3mm which are brass, and not a C bearing either…
TL;DR the Pixy still has the crown for smallest diameter C bearing, I’d say. And by a lot.