I agree that you need to remember that the weight is really just an estimate. Who can tell how much your baby is going to grow in the remainder of time?

Besides, not having had any yet, this is probably a bit of an ignorant assumption, people who have had babies of different weights will be able to tell you more accurately, but really, 7 pound or 9 pound, a baby is still a bigger thing squeezing through a smaller hole and how much difference will it really make?
I would imagine things like baby's position, our birthing position and pain relief techniques would affect the pain/difficulty factor more than size?

Someone correct me if I'm wrong, and of course, I may kick myself for typing all this if I happen to push out a 10 pounder