1. Your Diet
2. Your Training Method
3. How much you Rest
Increasing any body-part size depends on the method you use to point tension on that part. More tension means more chances to increase size.
Here I'll talk about Biceps. Our body is not apart into different organs. It's a combination of this parts. When you train your Back, your biceps and forearms already get exercised and no need to do any isolation Biceps workout. You do all exercise with your arms except Legs exercises.