Ds1 was 9kgs at 5.5mths old. Ds2 had almost dropped of the bottom of the chart at the same age. The paed watched his weight gain and decided there was nothing wrong but that he was just a slow weight gainer. This could be the same with your Dd. If she is happy, sleeping well and her output is good then I'd say she's just one of those kids who gains weight much more slowly. Ask booth yours and your DH's parents of either of you were slow weight gainers. I was a very slow weight gainer as well. Paed said it was constitutional rather than because of anything sinister.