It is possible to build muscle using low weight, provided that you train with enough volume and intensity.
There is a common misconception that high reps are better for improving definition. The only way to increase muscle tone is to get leaner and to increase muscle development.
Performing sets above 20 repetitions are probably too high for any significant muscle growth.
Low repetitions with heavier weight will force your central nervous system to work more efficiently at producing force. This is how you produce a lot of power.