They're still growing so I wouldn't worry about her weight at this stage. She could have a growth spurt in a few weeks and then you'll be worried that she's underweight!
When the boys were growing up I would often think that they were underweight (Riley looked like a string bean at one point, ridiculously lanky) and then a few weeks later I'd worry that they were putting on too much weight. They're 1 1/2 now and Riley is much heavier than Lu but that's down to his build, neither of them are overweight.