Most programs that help you build muscle size are focused on volume and high reps...typically with isolated exercises.
You get lots of time under tension, and lots of tissue breakdown.
The thing is...with these programs, if you only work in the 8-12 rep range, you may swell up -- but you won’t maximize you hypertrophy potential.
This is because the higher rep ranges are great at stimulating sarcoplasmic hypertrophy, (an increase in the fluid volume of the muscle)...but not so great at building myofibrillar hypertrophy -- an increase in the size and number of the actual muscle cells.
In order to build muscle that is big and full looking, as well as hard, dense, and strong...you need BOTH types of stimulation...which 5x5 hits so well.