some crazy people might tell you that muscle definition is a function of body fat (which you regulate through diet/exercise) and not muscle mass.
these same people might also tell you that trying to isolate the outer portion of your biceps is unlikely to yield results, and that you should focus on working the muscle as a whole (various curling movements) along with incorporating major lifts (bench, squat, deadlift, etc...) and an anabolic diet to successfully increase muscle mass.
personally, i think that's all wacky. if you want more definition on the outside of your biceps, do high-rep sets of 30 or more. oh, and face north when doing curls. always face north. exercising in any other direction makes you fat.