I like Deb K's opinion. My baby boy is also on the small side and for quite awhile, I've been stressing about how little he eats. Plus at our 6 month dr visit, the doctor said he was in a low percentile and we should work on getting his weight up. We've been putting formula into the breastmilk - along with as many solids we can feed him. He's been gradually gaining, but not as much as other babies. Now, I'm convinced it's just his genetics. Both my husband and I are smaller framed people and neither of us are heavy. I even went up to the hospital and pulled my pediatric records to try and get my growth charts (which weren't in the records I received - arrgh!) But I finally was able to get my baby book out of storage and my baby weighs more than I did at his age. I also was not a great eater. So I think if she is eating well, is happy, and healthy otherwise, than maybe you and your doctor should look into other reasons for her small size instead of trying to shove her full of even more food (besides, you can't force feed a kid, and I don't think it would be healthy to do anyways). Growth percentiles are just guidelines, and not every kid is going to fall into the chart. I don't necessarily think that means that anything is wrong. It's just an indicator for the medical profession to keep an eye on things and make sure the parents are not neglecting the child or anything like that. But I also fear that we may end up with the label, so I understand how you're feeling.
It's funny in a way, because the rest of America is doing what they can to fight childhood obesity, and here I am, trying to fatten my kid up!