A few years ago, I believe it was during the winter of 2001, I was executing a military press at the gym when my spotter suddenly stepped away from the bench, and began squinting his eyes while staring at me in the mirror. He looked deeply offended. I wasn't sure what I had done, until he eventually tilted his head slightly to the right, pronounced "hm," and notified me that the offense lay in my appearance. My left deltoid and trap were larger than my right. Sure enough when I examined myself in the mirror I noticed the difference. I was surprised I had never noticed the variation on my own. I inspected my left arm and quad and found them to be larger than their counterparts as well. I decided to test the strength of the muscles on my right and left side and found them to be comparable. So my left side was bigger, but not stronger. Hm indeed.
Symmetry is hard to achieve and maintain for body builders, but I'm an average sized guy who does weights here and there. I pondered this for a while and thought back to my exercise routines and -- reflecting on my ignorance -- I came up with a theory about what may have happened. During the first few years that I exercised, I consistently worked on the right side of my body first. During every set of bicep curls, back rows, or lateral raises I would work my right side, and then my left. I think the muscles on the left side of my body were enduring more stress because they were always worked second. The less oxygen a muscle has available to it the harder it has to work, and being that my left side was always going second it received less oxygen than my right side.
I spoke with a masseus hoping she might have some insight to this problem (similar to my expectation that Melinda holds the skeleton key to all medical doors because she is pre-med), but she wasn't sure what was going on. Paddling on the right side probably exacerbates the problem, being that the left side of the body is responsible for repeated lifting with the shoulder muscles, while the right side is responsible for repeated pulling with the back muscles.