When you lift weights and your muscles grow, all the tiny parts inside the muscle cells grow together in balance — so the overall mix inside the cell stays the same.

When you lift heavy weights regularly, your muscles get bigger because the individual muscle fibers inside them grow thicker and more numerous.

When new people start lifting weights — whether with light weights and restricted blood flow or heavy weights — their muscle fibers grow evenly inside, keeping the same balance between the parts that contract and the rest. This means their muscles grow the usual way, not by just filling up with fluid.

People respond to weight training in wildly different ways—some grow muscles a lot, others hardly at all—and those differences between people are about 40 times bigger than the tiny differences you’d see if you trained one leg harder than the other in the same person.

Even if you tweak your weightlifting routine—like lifting heavier, doing more reps, or resting longer—it won’t make your muscles grow bigger than a simple, steady progress plan, even if your body is making more muscle proteins.

Lifting weights makes your muscles grow because the longer and harder you work them, the more they get the signal to build new muscle fibers.