The Bulgari Octo Finissimo movements really do fill up the case, so it'd be hard to make the case much smaller. They probably wouldn't make it smaller by 1mm, they'd probably want at least a 3mm reduction for the trouble. The proportions as it is right now is quite nice. Have you tried the watch on your wrist? I have a very small wrist and not a large flat surface, but it works well on my wrist because the bracelet is quite supple and the links curve quickly enough. For instance, the bracelet on the AP Royal Oak is less supple and doesn't curve to a strong degree, so sometimes it flares out of my wrist a little bit. Main thing is to try it on the wrist. You might like it! I'm accustomed to 40mm watches on my small 16cm circumference wrist. Watches in the 40mm range have a generous look, but don't usually look comically oversized. I generally lean towards thinner watches, and the Octo Finissimo is a great piece for me.
I think you're right, the ROMA certainly has more possibility to reduce the diameter by 3 mm. But then the thickness might look ill proportioned to the diameter. I don't know and can't say for sure. But it's definitely a very nice watch.