It mostly has to do with positioning. A malpositioned babe, e.g. an OP (occiput posterior), will give you a much more painful, and longer labor than a properly positoned babe. Size has nothing to do with it.

I know this from experience. My first babe was OP and I had an excruciatingly painful labor, which included back labor, a special horror you don't want to experience!

My second babe was over ten pounds, but properly positioned, and it didn't hurt all that bad at all, and no back labor pain! And it was faster.

Every Momto-be should read this website:

www.spinningbabies.com

Also, try to avoid an epidural if possible. It slows your labor dramtically and increases your chance of a c-section. Labor pain is completely doable. And it stops the minute the babe is born.