She's a tiny little girl. Yes, use TINY treats. I like Zuke's and still break them in half (3 calories per treat). I also like the soft Buddy Biscuits and break them up easily. My boy is 16lbs but I still make them very tiny.
I use treats while training, which for us, occurs at least 5 days a week. But he gets a chicken jerky strip every night before bed (I only buy the kind made in the USA - don't buy chicken jerky from China!) But it's always very tiny pieces as to not fill him up and keep him interested in working.
I don't really like milkbones, dog chow, etc, very much due to the ingredients and the recalls that have been involved. But you can do your own research if you'd like to find out more about that.
Good luck! I'd love to see pics of your baby. Oh, as for growth, sounds normal. My boy is bigger so it's hard to compare, but he was 6lbs at 12 weeks, 8lbs at 18 weeks, 12lbs by 6 months, and now 16lbs at 3yrs old.