Making your pitbull muscular is a fine balance between exercise and diet. The appropriate diet will not be too fatty and should contain a good amount of protein to aid in recovery. You will also want to feed them little and often between training sessions to avoid having your dog gorge itself, potentially running the risk of sickness. Furthermore, you will need to involve them in a rigorous exercise regime such as agility training in order to gain the full benefit of their exercise and achieve the muscular look you are looking for.