One bicept/tricept bigger than the other?

My left arm is about 1 3/4" inches bigger than my right arm, this problem has me a little worried. I have been working out with machines at the gym for about 7 months now, i think this is the problem. I started working out with dumbbells about 2 months ago. I have been reading up on the subject and i am doing an extra set on my left arm when i do free weights. Anyone have any good tips on how to even out my arms?