The 28 Ultron is a little larger than the very expensive Leica 28 f/2 Summicron, and the new Leica 28 f/2.8 Elmarit ASPH is smaller yet (the previous non-Asph Elmarit - another choice - is the same size as the Summicron). And the Voigtlander Skopar 28 f/3.5 is even smaller, very affordable, and excellent optically and mechanically, a little gem, but a bit on the slow side. Discontinued, but you can find them available.
I am not familiar with the older CANON RF 28mm lenses, but that's probably another option.
"Fast" was a priority for me, and size also was a priority, so it really boiled down to only one possible unaffordable choice for me: the 28 Summicron Asph, the best combination of fast and small with excellent optics. Stalemate, until I found a new one, old stock at old price, from a dealer clearing his dusty shelves. It was still expensive, but I couldn't resist. 🙂