Squats. Deadlift.

I wouldn't worry. I have some minor "cosmetic" imbalances (left calf smaller, left pec used to be smaller, left hand is weaker at CoC's, etc). It doesn't bother me. None of it really affects my performance, so I don't care. I was also sorta serious with the heavy lifts. I found they help somewhat. Your "weaker" side won't stay weak and provided you're using free weights you are forced to use both sides when you lift.