Since training with weights I always concentrated on arms and wanted the bicep vein all the time. Still to this day I train my arms hard and want to improve them all the time. However an issue that I have and I know that most the guys in the gym have is size difference in their left to right biceps.
So how can we fix this?!?!?!?!? Well for one stop flexing so much into your dominate arm (I know you do at home as everyone does).
You can also add more sets at the end of a bicep routine for the weaker side or you can add a unilateral bicep exercise and focus only the weaker hand side. The other strategy is to add a bicep exercise to another workout (a couple days away from the main bicep workout) and only focus on the weaker side or last is have a separate day in your week just focusing on the weaker bicep.
Even during a two handed exercise I would also recommend that you focus more on the weaker side during a workout. What I mean by this is in a two handed (barbell) or one handed (dumbbell) exercise watch / focus your energy into your weaker side.
Bicep is probably the most common muscle to have a massive lag in size and mass but these ideas do apply to any other muscle group in the body.
With these strategies this can help you bring up that lagging bicep (or other muscles) and balance the muscle size issues. So for some motivation I though I would post some photos for motivation, these are probably the two great bodies in the history of bodybuilding and that is Arnold and Frank zane