Even if one exercise uses more muscles than another, both can make the same muscle grow just as big if you train hard enough.
Evidence from Studies
Supporting (1)
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Hip thrust and back squat training elicit similar gluteus muscle hypertrophy and transfer similarly to the deadlift
Even though squats work more muscles overall, they didn’t make your butt grow bigger than hip thrusts did—when both exercises were done the same number of times and with the same effort.
Contradicting (3)
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Comparison of Muscle Hypertrophy and Strength Adaptations Induced by Back Squat and Leg Extension Resistance Exercises
The study found that doing leg extensions (isolating one muscle) made the front thigh muscle grow more than squats (which use many muscles), even though both workouts were equally hard and had the same number of reps. This means using more muscles at once doesn’t always make your muscles grow bigger — so the claim is wrong.
Cluster sets and traditional sets elicit similar muscular hypertrophy: a volume and effort-matched study in resistance-trained individuals
This study didn't compare big multi-joint exercises to single-joint exercises — it just compared two ways of doing the same exercises, so it doesn't tell us whether compound moves build more muscle than isolation moves.
Volume-equated high- and low-repetition daily undulating programming strategies produce similar hypertrophy and strength adaptations.
This study found that lifting heavy and lifting lighter weights — both using big multi-joint exercises like squats and bench presses — built muscle just as well when the total work was the same. That means big exercises aren’t worse than single-muscle moves for growing muscle, which goes against the claim.