The stores around here will cut a butt in half to portion it out and charge a higher price for the extra processing. Next time look for a cryovac bone in butt in the 7.5 to 10 lb range. After cooking, you will lose close to half of the weight, so it's not as much meat as you think.
If it didn't pull easily, it wasn't quite done. They turn to mush when overdone, and don't pull easily when underdone. The bone is the built in thermometer. When bone wiggles freely, and almost wants to fall out, it's done. And don't forget to rest for at least an hour before serving, I prefer 2-4 hour rest myself.