As training progressed he became stronger. he could not do pull-ups at first and had to use bands and an assisted pull-up machine at the gym.
Diligent training with combined nutrition caused him to get stronger. He is now doing pull-ups with 70 lbs addition weight hanging from his waist!!
Now the idea most of us have, the stronger you become the more muscle you gain, while that has some truth to it , it might not necessarily get you the body and look your are trying to achieve.
Your body is extremely adaptive and your existing muscles can become stronger, build more efficient neural pathways to execute the motion without you seen a big change in muscle shape or mass.