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It could also be that you're less coordinated in the left side and can't flex the muscle as well as on the right side. I had this problem with my left bicep. For ages I thought the left bicep was smaller, but I eventually realized I don't have the same muscle control to tense it as hard as the right bicep. I check the difference in poses that don't require flexing each arm independently. For instance, I put my arms overhead, flared my lats, and clasped my hands together and put pressure against each hand, which flexed each arm indirectly. I noticed my arms were virtually identical. Same with when I was doing incline chest presses, I could see at the mirror as my arms tensed that they were equal size, definition.
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