I think this is something every mom worries about at some point.
http://www.cnn.com/2008/HEALTH/family/03/12/par.so.big/index.html

Oh yeah! I was really worried about Jack in the first 6 months of his life. That's because he was very small. He was born 7 lbs. 1 oz. but then he gained weight slowly and his weight percentiles were barely making the charts. He was in the 5-10th percentile at one point and a doc in Florida (where we used to live) said if he didn't start gaining more we'd have to start doing testing on him. EEEEEEK! Of course I was a basket case trying to think of what ailment he had.
Then we moved here and my doc here said kids are all different sizes and not to panic. Jack was gaining, just in his own time. And he was a breastfed baby which probably explained why he was gaining in smaller intervals than his bottle-fed peers. Well, he's now in the 30-50th percentile and doing just fine. So all that worrying for nothing!

My problem is exactly the opposite of yours Kristi. Ronald was born at 11 lbs, 4 ounces, and his first doctor kept calling him fat. I had gestational diabetes, and so he got bigger than he would have. I still think he would have been 9 pounds or better even without the diabetes. I have since changed docs, and taking in my family genes(my dad is 6'6" and grandpa was 6'11" tall, and my DH is 6'3") she said even though he is in the 90 percentile for height and weight, as long as he doesn't start to get unbalanced (weight bigger that height) that I shouldn't worry, that he is just going to be a big kid. I still can't help but worry, but I try not to let it get the best of me. We just try to think that our son will be the biggest kid on the playground and won't have to worry about being bullied. LOL!!