How to Flex Pecs

When flexing my pecs, one seems to have more skin than the other and makes my chest look uneven but only when flexing, how can this be fixed? I weightlift most days.

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Leaf Daddy's Advanced Lesson: The Pecs Flex by kusanagi_artist -- Fur Affinity [dot] net

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How do you contract certain muscle fibers when flexing the Pectoralis Muscle like this guy? I can flex mine, but not certain small groups of muscle fibers. : r/bodybuilding

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Terry Crews, Jimmy Fallon Flex Their Pecs to the Beat in 'Nip Sync Duet' (Video)