I'm with Ash and wanna kick him/her, lol.
My sisters pelvis was too small to deliver her girls but both times they let her try naturally first.
A c-section is major abdominal surgery and you want to avoid it at all costs, i say go naturally and if a problem arises and u have no other choice then so be it but best to give your body a go at it first. I'm small and i had a 8pound12.5oz baby.
As for the weight estimate, my friend was told her baby was going to be huge (9pound or something) and she had a 6pound something baby. They are highly inaccurate.

Good luck and fingers crossed that you have a natural birth