Janus - silly question maybe, but why do you want her to 'fatten up'?
The reason for my question is that my dd is also small, weight wise, and always has been since about 2 mths old. She too was in the red zone, and my HV suggested weaning at 14 weeks, and then very helpfully suggested fattening puddings etc later on.
She is now 21/2, still light and slight, but eats a very healthy diet, has lots of energy and vitality, and is fine. She too eats very small portions of food, and has been, up until recently, very fussy with food.
Surely it is more important to give your daughter a healthy diet, bearing in mind their requirements at this age for full fat etc. You are doing the right thing by ignoring the charts.
Keep her diet healthy and balanced, and she will settle at the build that she is supposed to be.
P.S. Dd can't wear trousers because they dont stay up, she has no waist, so she wears lovely dresses. Bonus!