If you believe she is eating reasonably well, and are always offering her food - then I would not stress about it too much. By all means keep an eye on her weight and keep offering her food, but some kids are just smaller.

My two are as different as can be. My DD is a 'boof' - she is huge (tall and well built, but not overweight for her height), she has always been in the top 5%. My DS on the other hand, is 21mths old, 10kg and just below average in height. He is skinny, but what can you do? I cann't force him to eat, and he is happy and healthy. I sometimes worry about his weight and height, but in the end all kids are different.