I think this is such a hard question. My daughter was never a great eater. She never drank that much milk (about 25 oz a day at her max) but did not want any more than that. I had more available (could always pump extra) but that was all she wanted. She fell in her % from 10% at 4 mo to 7% at 6 mo to 3% at 9 mo to less than 3% at 1 year and that is where she has stayed. But when I looked at the curves for bf babies on kellymom.com, she followed a typical bf baby curve. It was very hard not to stress about it. That being said she was always gaining, just very slowly. I had a scale that I weighed her with before and after feeds, so I knew how much she was eating. I also couldn't "force" her to eat more when she wasn't hungry.
When we introduced solids (at 6 mo), she didn't accept them until she was 7.5 mo. Then she would go through periods where she accepted them happily and times she wouldn't. We also tried to add in calories where we could (butter, avocado, yogurt, etc.). Remember babies need fats for brain development. I think if your doctor is not concerned, then I wouldn't be either.
My daughter didn't eat well and at 12 mo we started seeing a feeding therapist. She has made tremendous improvements and now at 18 mo she no longer needs the therapist and is eating a pretty good variety of foods. She is still pretty tiny, but is growing well.
Good luck!
Jennifer