Pitbull Question....?

I rescued a 5 week old pitbull pup from a guy that had a litter but did not take care of them. Out of a litter of 9, my pup is the only one that survived. The mom weighs around 50lbs, and the dad is 65lbs. The pup was on the verge of death, loaded with coccidia, and other intestinal worms. I work at a vet, so i treated her for all of that, and got her all up to date with her vaccinations. My pup is now 16 weeks, and weighs only 15lbs. Everyone makes fun of my midget pitbull as they say, but shes perfectly healthy now. I exercise her daily and shes lean and muscular, but i was wondering on average how much do you think she will weigh when shes fully grown? I dont care if she is tiny, because shes full of energy, healthy, and happy, but I was just wondering out of curiousity. Any information would be greatly appreciated!