I think growth rates in the first 6 months are very dependent upon feeding success. I had very low milk supply with both my children, despite seeing several lactation consultants. Both of my sons were born at 50% for height and weight. My first son just wouldn't gain weight from breastfeeding alone -- I tried, under doctor supervision -- for a month. Finally, we started supplementing, and then he stabilized, and has always remained between 50-75% for height and weight (he's now 5). I was able to exclusively breast-feed my second son, but when you look at his height and weight curves, you can see that my low milk supply was definitely keeping his growth on the low end. He was born at 50%, then slowly but steadily dropped to 10% by 4 months. At 4 months, I started introducing solids, and his height / weight crept back up to 25%. He stayed in the 25% range until 1 year of age, when I stopped breastfeeding and switched to whole milk ... and within a couple months his height / weight soared to 75% which is where he still is currently (at age 3).
And, although breastfeeding tends to result in slower growth, its not always the case. I had a friend who's baby was born at 50% and was exclusively breastfeed and within a few months he was at the 99.9% percentile for height and weight. Both parents were tallish, but not unusually tall, and the mom did say that she produced a LOT of milk. He's 5 now, and is still a BIG kid (all around, not fat).