Just to give you some kind of comparison, My 13 wk Boston female weighs exactly 8lbs. I recently visited the breeder that I purchased her from, She still have 2 of my pups litter mates. One was very much smaller then my girl, and the other was bigger.
My two year old boston is 21 lbs she is very healthy and my 11 mo old is 12lbs and he is the runt. It is all about what you feed thme