My little Paddington sounds just the same don't panic. He's a purebred miniature schnauzer too and 14wks now, and as lankier than you would believe, he's been getting taller and taller, the vet promises me he'll start filling out soon! he weighs about 5 kgs now, But I think males are probably bigger than females. What colour is your little girl?
Jetta is a purebred little black girl & we just love her. I really like the other colours but love the black ones. She has such a fantastic personality and she is a little spoilt. I am so happy I chose a Miniature Schnauzer as I am sure you are too. I did alot of research on the net about them before I decided to get her but it only ever mentioned what height and weight they should be as adults. Going on what you have said about Paddington I think she should be at least 4kg by now.