Hard to tell yet; my baby is only eight weeks old and was born two weeks before her due date. She's small for weight (25th centile) but then her dad is tall and slender (more slender now he's primal!).
But she was holding her head up (albeit only for a couple of seconds) from birth and can wriggle with purpose: if she's given a leg up she can crawl from my belly to my shoulder, and then flip herself into the crook of my arm into a feeding position! She smiled to herself from day two and is now giving proper social smiles to us.
The growth charts we're using here (I don't know if yours are the same) are apparently based on healthy breastfed babies from several countries, so the local diet should get at least a little cancelled out. Nevertheless, it's possible that they're overestimating the desired weight of babies by using a sample size which is in reality skewed towards higher weights.
t's also worth bearing in mind that more active babies will be naturally slimmer, for obvious reasons! As long as she seems healthy in herself and is meeting developmental milestones, don't let the doctor's worries concern you.