Also consult a chiropractor. Imbalances in the spine can cause nerve issues which subsequently cause strength discrepancies between the left and right sides of the body. Before I went to my chiropractor, for instance, my left hand grip strength was 7lbs. weaker than my right; after a few weeks of treatment, my right hand grip strength improved by 3lbs. and my left hand grip strength was only 1lb. weaker than my right.
Strength imbalances aren't necessarily due to one muscle group being smaller than another; nerve control is important to consider as well.