Right fucking answer.
There's multiple factors at play here.
Have you had prior injury which made you favor one side?
Are there muscular imbalances, as mentioned by lift206, with stabilizers and the less glamorous muscles very few people care about (even though they should)?
Do you have good MMC in the weaker/smaller muscle group?
Are there trigger points effecting proper mobility? I read in another thread about how you recently got a massage and felt much better afterwards. Adhesions in the fascia can inhibit proper scapular movement. This, in turn, alters how your muscles respond to a given task. Inevitably, one side becomes dominant and more neurally efficient. This, obviously, results in a visual disparity.
And it's not enough to just toss in some extra unilateral work as an after thought. It's fine to do some activation work on the weaker side on your off days or before your main lifting session; but you do need to prioritize them. You need to attack those issues pissed off and ready for war.