If you're a young guy who's never lifted weights before and you do pec deck exercises, your upper and lower chest muscles will grow the same amount—no one part gets bigger than the other.

If untrained young guys do pec deck workouts three times a week for three months, following a specific plan, their chest muscles get noticeably thicker—by about 17 to 21%—in both the upper and lower parts of the chest.

If you're a young guy who hasn't trained before and you do pec deck exercises for 12 weeks, your chest muscles will get thicker—about 17–18% thicker in the upper part and 21% thicker in the lower part—no matter if you do just one set or three sets each workout.

If you're new to working out, doing just one set of chest machine exercises three times a week gives about the same muscle growth as doing three sets — both lead to around a 20% bigger chest muscle after 12 weeks.

If you're new to working out, both parts of your chest muscle might grow about the same when you lift weights.

If you're a young guy who hasn't lifted weights before and you do pec deck exercises for 12 weeks, you'll probably get about 43–46% stronger, no matter if you do just one set or three sets each time.