Training one arm or leg at a time doesn’t make muscles bigger than using both at once, at least for small muscles like the biceps.
Evidence from Studies
Supporting (3)
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Small muscle mass exercise enhances muscular adaptations? Effects of unilateral and bilateral biceps curl on maximum strength and muscle size changes.
Both ways of doing bicep curls—using both arms at once or one arm at a time—built the same amount of muscle when people did the same number of reps and worked just as hard.
This study showed that training one arm or leg at a time with the same amount of work leads to big muscle gains, just like training both sides together would—so the claim that one-sided and two-sided training are equally good for building muscle (when you do the same total work) is supported.
Neuromuscular Adaptations to Unilateral vs. Bilateral Strength Training in Women
Both one-arm/one-leg and two-arm/two-leg workouts made the muscles grow the same amount, as long as people did the same total amount of work.
Contradicting (2)
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The Effect of Unilateral and Bilateral Leg Press Training on Lower Body Strength and Power and Athletic Performance in Adolescent Rugby Players
Even when doing the same amount of work, training one leg at a time made that leg stronger than training both legs together — so they’re not the same.
Even though both types of workouts were done the same number of times, one type made some leg muscles grow more, and the other made different muscles grow more — so they didn’t have the same effect.