I can attest to this point Jack. my left arm is weaker, however it has a more "full" muscle belly, higher peak, and is wider. My right arm my stronger arm but the muscle appears inferior. however upon closer inspection its plain to see that although the muscle may be slightly smaller overall, the biggest difference in perception comes from the way that the muscle is dispersed on the frame of the arm. its longer and therefore thinner and flatter.
Its the same principle as 2 200lb bodybuilders standing next to each other and one is 5'6" and the other is 6'0". the shorter guys are going to look bigger everytime...