I think this is normal, although I do not have alot of experience. Our boy is 6.5m old and he is about 18lbs at this point. He is still small in comparison to adult dogs. So I wouldn't worry too much, there is also a discussion on height/ weight of pups somewhere on there (I forgot where)
I think I remember reading that pups reach their full height at anywhere between 6 and 8 months. Maybe she'll do more of her growing toward the end. Our (male) pup on the other hand is 6 days short of 6 months and probably pushing about 22 lbs, and now rivaling our adult female in size and speed for sure. So I'm thinking he grew up pretty fast and should plateau soon. Or we'll have a huuge basenji. Nulla's a sweetheart, I love her face
My little girl, Lexi weighs between 18 and 19lbs (feautured to the right of Miles here). She's seven. She has a smaller body and head too. Some are just "petite".
I have opposite ends of the size spectrum with my kids… Lexi's a tiny girl and Miles is a huge boy (34lbs).
I don't think you have anything to worry about. She's such a cutie pie!
If you have not done so yet - ask your breeder what he/she thinks. Some lines are slow to mature - some lines have smallish basenjis as normal. In any event many breeders would want to know how their pups are delevoping so contact them as they would have the best and most specific information for your dog.