Agree with OldTimer1, for the most part. It all starts on the individual muscle cellar level, and is quite complex. The goal is to have all elements balance with one another: DNA/RNA, amino acids,body chemical restructure/designs, absorption, waste removal, etc.
Very old saying among BB'ers; "More blood, more muscle". Volume (getting and keeping the pump) has been king for a very long time. Some of the BB'ers of the late 60's, 70's and early 80's performed higher rep's (10 to 15, perhaps more) workouts, 20- 30 sets a body part. With the current drug use, that volume can be reduced by 50%, perhaps more for some men. Drugs influence recovery quite quickly and getting the body ready to adapt to the next workout. Side affect; some BB'ers appear bloated and soft, in normal everyday life.
Should be getting real powerful with set's of 2's to 3's, which tends to produce dense tissue (though not the greatest method for inducing max muscle size) , along with thicker muscle attachments. A higher rep range between, 5 & 7 rep's, will improve strength and muscle size. Heavy lifters tend to have a thicker appearance than BB'ers. Some of them look like walking through a wall would be no big thing. On the other hand, BB'ers have to open a door to get through to the other side.
There is also Power BB'ing, which can give some guy's the best of both worlds; being very strong, with noticeable muscle mass. These lifters do use heavier weight when doing the classic BB'ing exercises. Cheating is welcomed in this style training.
As OldTimer1 stated, always a good idea to change the workout style around, from time to time. Go heavy, go moderate or even go light. After all, lifting is one be experimental lab, you never really know the results you may get, if you don't try all phases of workout protocols. Good Luck.