Your pediatrician should be charting her, and if she is "thriving" she'll stay on the same growth "trajectory" irregardless of how she compares to other children.
For example, my daughter was 40-50th percentile in weight the entire tracking. She didn't reach 20 lbs until she was around 15 mos. My friend's daughter was also breastfed but was in the negative 5-25% her entire tracking. Both kids healthy.
Conversely, my son was also breastfed, on the same percentiles, but reached 20 lbs at 10 mos...Here's a good link for comparative between breast and formula growth charts.
http://www.kellymom.com/babyconcerns/growth/growthcharts....
It was such a drag to wait and change her seat forward:) Hope this helps:)