It's normal for one side of your body to be more developed than the other. There are a few reasons why, these are the two I can think of off the top of my head-
1. You have stronger neuromuscular connections to the dominant side of your body. This means that if you're right handed you can lift more with the right side of your body. Because you can recruit more muscle fibers on the right side of your body you'll be putting that side of your body under more stimulus thus more reason for that part of your body to grow.
2. People use their dominant side of their body more often. Once again, more stimulus on that muscle means more muscle growth.
3. if you do isolation exercises then you may further exxagerate the emphasis on one side of your body.
Almost everyone has more muscle on one side of their body than the other. It's natural. Even look at the pro's who work day and night to achieve perfect symmetry. No one's perfect, so don't sweat it if one arm is a little bigger than the other, it's normal.