When we increase load or weight our muscles get stretched quickly but level of fatigue is high at the maximum and we cannot perform much repetitions. In contrary, stretching of muscles is a bit after much repetitions achieved when we use less weight or load as level of fatigue is low even at the same maximum. First, we used more weight and less repetitions to come to a fatigue and second we used less weight and more repetitions to come to same fatigue level. You can see, same maximum stretching or fatigue level can be achieved with more and less weight or load but things are different as cleared-up above. Another important aspect is, while performing more repetitions with less weight or load, you give more pressure or effort to your muscles thus your muscles get more pumped or have better blood flow and vice-versa. Also, your body is in easy and safer side while performing high repetitions with less weight or load that helps hypertrophy. In contrary, your body is in difficult and less-safer side while performing less repetitions with more weight or load that does not helps hypertrophy much.
Performing exercises with very less weight or load will not be effective and efficient so may prove just waste of time.
Thus, in a conclusion, performing exercises with medium weight or load with proper technique, posture and programming will prove fruitful in all respects and you will achieve your goal of muscular hypertrophy in stipulated time frame.
