The likely reason of them imbalance (as you say) is the preferential use of the dominant shoulder. The other reason for the same can be the pain that you have under the scapula. Pain usually leads to disuse atrophy. Please consult your orthopaedic surgeon for the likely cause of pain and atrophy of the concerned side. A brief history clinical examination will reveal the likely cause of symptoms. Nothing to worry about but needs consultation if it is troubling you.