Well I think the body is adaptive in two ways.
1) Progression of weights - Lifting heavy and trying to keep moving up
2) Progression of reps - Stamina if you will, your body can do more reps if you push it.
I think both can build size. Maybe reps are more for tweaking/sculpting (?).
I go back to a bicyclists legs. Some of them just look plain funny with their frail upper body and their tree trunk legs. Do you think if they were squatting heavy that their torso would look like a twig? I don't. They may be squatting, but I feel sure that it is light to moderate weight for reps. But I could be wrong.