My left pec is bigger than my right, my left tricep is bigger than my right.
So, will I think this is genetics, or maybe the fact that theres a strength/form problem?
Well, tri's and chest are hit heavy with barbell bench pressing. If my right arm is even SLIGHTLY stronger than my left, then my left arm will actually be adding more muscle, because it struggles with the weight. You get to the end of your last set and my left has worked harder than the right, so more hypertrophy occurs on the left [those last few real muscle-building reps], but strength on my right still goes up. Unless I do dumbell work to even out the strength imbalance, there will always be this problem and it's just begging to turn into a severe injury.
So I go through the stages of building identical strength in all my muscles, and I go back to BB bench presses. not only am I lifting 10-15KG more, but I feel the form is much better. After a while, my muscle size still hasnt caught up so I put it to genetics.
WRONG!
Two to three years later, I notice that my pecs and tri's are of equal size. Why was this? Strength isnt directly relational to hypertrophy, so a noticable size difference between the left/right pec/tri has evened out over time.
I think we can all be pretty symetrical with a lot of hard work and time. Sure, we wont have the symetry of ronnie coleman or many of the other Pro BB'ers, but I dont think anyone should settle for a noticable larger tricep. Ever. Dont say it's genetics until you've put your balls into it for a long time.