I have two friends who's children had some issues when born who were much smaller at 3 months that this child. One was my assistant who's child was alergic to milk. The attempts to find a suitable food for her were very frustrating and she grew slowly, but she did grow. Once they found out the problem she grew faster. (She was alergic to any milk, including mom's and soy milk). She seemed like a happy baby but did have an upset tummy quite often. One would look at her and never know she had problems if they didn't hear her when her tummy hurt.
The other was a small baby and still is a petite little thing, but the healthiest little elf you ever could see. She came by to visit last night and wore out my kitties chasing them around the house. She's tiny, but very very healthy!
At six months I barely topped 5 lbs by the way, and I seem to be somewhat ok now (Paragon would tell you I'm a flaming liberal so I'm not entirely ok)
If the baby is eating, happy, and you can assure it's in a healthy environment, I wouldn't worry too much