Most people lift for a specific reason. Athletes train to get better at their particular sport. Bodybuilders train to achieve a symmetry rarely found in the human form. (Although lately, to me, it seems it's just to get as freaky big as possible!) Powerlifters train to lift more in the big 3, squat, bench and deadlift. I think most guys you see in the gym don't fall in any of these categories. They lift because they want some bigger muscles. Pretty simple. But they don't do the simple things correctly.
Lifting is a stimulus you do to create a desired effect. If bigger muscles are the goal, you have to lift to stimulate the correct hormonal response. Hormones drive the car being built.
In a paper called the Salivary Testosterone and Cortisol Response to Three Loading Schemes. Three methods were tested using weight trained males in the squat.
1. 8 sets of 6 reps @ 45% of their 1 rep max. 3 minutes of rest. Weights were lifted ballistically (FAST)
2. 10 sets of 10 reps @ 75% of their 1 rep max. 2 minutes of rest. Weights were lifted under control.
3. 6 sets of 4 reps @ 88% of their 1 rep max. 4 minutes of rest. Weights were lifted with explosive intent.
Time of each workouts were kept to pretty much the same. Salivary Testosterone and Cortisol were both elevated following the 10x10 workout. No endocrine (hormonal response) was seen in the 8x6 or 6x4 rep schemes.
So, from the data of this study it looks to be that the big variable you must manipulate to get bigger muscles (hypertrophy) volume . Charles Poliquin calls his version the German Volume Training method. I just have to warn you, 10 x10 @ 75% with 2 minutes rest is one HARD workout. But, that's probably why not every guys walking down the street is jacked!