A study from a few years ago concluded it's intensity of effort, as in going to failure, that makes or breaks a set for hypertrophy. Whether that was for 5, 10, 20 or even more reps made no significant difference.
That also explains why cyclists often have huge legs. I'm sure if you'd put them on pro bodybuilding cycles and continued to let them cycle all day, they'd have legs as big as IFBB pro's.
(no, pro cyclists aren't blasting 5gr or more of steroids per week)