Resistance training to volitional failure produces similar hypertrophy in both slow-twitch and fast-twitch muscle fibers regardless of load or repetition range.

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

This study found that lifting light weights with lots of reps and heavy weights with few reps both made muscles bigger when people pushed to exhaustion — so it supports the idea that how heavy the weight is doesn’t matter as long as you go all the way to failure.

Study: Resistance training load does not determine resistance training-induced hypertrophy across upper and lower limbs in healthy young males.

When people lift weights until they can’t do another rep, whether they use heavy weights with few reps or light weights with many reps, their muscles grow about the same — the key is pushing to exhaustion, not how heavy the weight is.

Study: The Effects of Low-Load vs. High-Load Resistance Training on Muscle Fiber Hypertrophy: A Meta-Analysis

When people lift weights to the point of exhaustion, whether they use light weights with lots of reps or heavy weights with few reps, both slow and fast muscle fibers grow about the same amount — this study found no real difference between the two methods.