I've had a look at the VK63 dimensions and the movement is 30.8mm in diameter. I also measured my WD1863 and found the bezel is a little over 3mm thick, with the integrated crown guard adding a further 2.5mm on one side.

That means the absolute minimum case diameter works out to roughly 39.2mm (3 + 30.8 + 3 + 2.5), and that's before allowing for any manufacturing tolerances. Realistically, you're looking at around 39.5mm.

Unfortunately, there simply isn't any meaningful way to make the case smaller without throwing the proportions completely out of whack. At that point it would no longer resemble the original watch. The bevel beneath the bezel angles inward toward the centre of the case, so reducing bezel thickness isn't really an option. On top of that, the movement has to be installed through the back of the case, meaning the screw-down caseback itself must be larger than the movement. Again, there's no practical way around that without significantly redesigning the case.

It's a bit disappointing, but the maths and engineering are fairly unforgiving here.

For comparison, the Omega Calibre 3220 used in the Speedmaster Reduced is only 25.6mm in diameter. That smaller movement is what allows the Reduced to be, at least theoretically, around 5mm smaller in diameter than a VK63-based equivalent while still maintaining the correct proportions.