I use a smaller wheel than in some IT cars but I have power steering.

It's less about diameter than it is about force. If you are pulling** on the wheel and need to make an adjustment, it's harder to modulate the change in forces you are exerting, if they are of a greater magitude. With PS, the forces required to move the wheel are low, so modulation isn't such an issue.

Once force is taken out of the picture, it is easier to have "fast hands" with a smaller wheel, since the movements are physically smaller.

** EDIT - you can see from the pic that we also sit close and LOW in this car, so steering inputs are like you'd expect in a Nextel Cup car, rather than in the "classic" arms-straight sports car kind of pose. THAT is arguably a much bigger issue, when it comes to smooth, accurate inputs. When I say "pulling" on the wheel, I mean just that. Just like with a wrench, you do not want to be pushing (away or up) on the wheel. You have WAY more control when you need to adjust.

K