One of the very best things (at least for me) about breastfeeding is that it takes the worry out of baby growth. Your milk is made specifically just for your baby and your baby's needs. So if your baby is bigger than others, than that is simply how she is supposed to be. She is growing into what God/Nature intended.
I had the exact opposite problem as you. I gave birth to a 9lb 1.4oz baby girl (gestational diabetes had made her fat fat fat). At her six week check up she weighed 9lbs even. I was scared to death and wasn't too reassured by a luke warm ped (thank god my ped was actually PRO breastfeeding, though, otherwise she probably would have had me put her on formula right away). A friend of my husband's coworker is a very probreastfeeding ped and he called me and spent over an hour talking to me about how breastmilk works and how it feeds the baby exactly what the baby needs. Even if the baby isn't growing the way we think the baby should. He told me if your baby isn't hungry, is wetting diapers, and is generally in good health, then you can bet she's getting enough to eat.
She's still a mini. At 4.5 yrs old she's 33lbs and wears shorts in sizes 18 mos - 2T (jeans have to be 4T slims and even those are baggy). Oh, and both her dad and I are ummm... large people. Always have been (of course, we were both ff as well so there you go).
Anyway, my point here is (and yes, I am long winded) that as long as you are exclusively breastfeeding, then you don't even have to worry about her weight. She's growing perfectly. Congratulations, Mama!