Very high-repetition resistance training with very low loads (e.g., >100 reps at 30% 1RM) produces muscle hypertrophy equivalent to that produced by moderate-load resistance training (e.g., 80% 1RM) when both are performed to volitional failure.

Study: Divergent Strength Gains but Similar Hypertrophy After Low-Load and High-Load Resistance Exercise Training in Trained Individuals: Many Roads Lead to Rome.

Even if you lift very light weights, if you keep going until you can’t do another rep, your muscles can grow just as much as if you lifted heavy weights. This study shows that both light and heavy lifting, when pushed to exhaustion, lead to similar muscle growth.

Study: Muscle Hypertrophy, Strength, and Salivary Hormone Changes Following 9 Weeks of High- or Low-Load Resistance Training

When people lifted very light weights for over 100 reps until exhaustion, their muscles grew just as much as when others lifted heavier weights—except for one muscle (triceps), which grew a bit more with heavy weights. So, for most muscles, light weights can build size just like heavy ones if you push to total fatigue.

Study: Low-Load Resistance Training to Volitional Failure Induces Muscle Hypertrophy Similar to Volume-Matched, Velocity Fatigue

This study found that lifting very light weights until you can’t do another rep builds muscle just as well as lifting heavier weights, as long as you push yourself to complete fatigue.

Study: Neuromuscular Adaptations After 2 and 4 Weeks of 80% Versus 30% 1 Repetition Maximum Resistance Training to Failure

This study found that lifting very light weights until you can't do another rep builds muscle just as well as lifting heavier weights, as long as you push yourself to complete fatigue.