There are two possibilities here. The first is that you are using your left arm and leg more than your right. This could happen, potentially, if you were left handed. In left handed people who are physically active, sometimes the left arm or leg will be slightly larger or stronger than the right arm or leg. Usually the difference is pretty subtle though.
The second possibility is something called hemihypertrophy. This is a genetic condition that some people are born with that causes one side of their body to be larger than the other. Although having this size difference in and of itself is not a problem, hemihypertrophy can be associated with other genetic problems such as heart problems. Usually, however, these are picked up in childhood, and if you have made it to adulthood it is less likely that you have another problem. Also, children with hemihypertrophy are at higher risk of certain types of cancer, especially kidney and liver cancers. Again, by the time you make it to adulthood, this risk nearly entirely goes away.
However, it is worth mentioning this to your primary care doctor
to see if they feel any additional workup might be needed at this time.