My little guy is on the 10th percentile, so he is quite little - BUT, he is the same weight and height as my DD at the same age and on the charts in her baby book they are on the 50th percentile - they have changed the charts in the last few years, so on the old BF based charts your DD wouldn't be so 'little'.

I've used BLS with both of mine and initially with DS, when I was getting comments about his size I was paranoid, I have to admit I did try some commercially prepared babyfood. He hated it anyway so I stopped.
He still eats pittance but same as your little girl, he's happy, feeding well and meeting all his milestones.
As long as she's still drinking well, food isn't their main source of nutrition at that age. Stand your ground. YOu are her mum. Some kids are just little.