Read Bob Smith's post, as he beat me to it for the most part. The only other factor would be the degree of sarcoplasmic hypertrophy compared to hypertrophy of the actual sarcomere (myofibrils, etc). The person with more hypertrophy of the sarcomere would have more "denser", "harder" looking muscles, assuming they were at comparable bodyfat percentage levels. Now, its not really fair to compare JS to someone a foot shorter than him, because we all know shorter guys (like myself) look bigger than we actually are. But considering JS trains in the smaller rep range (1-5 reps) as opposed to Lee Priest's 5-20 reps, than JS would have denser, harder looking arms. As far as who would have a better "peak" and all that nonsense, thats genetically predetermined based on your individual insertions and origins, tendon lengths, etc. You can't change any of this from doing concentration curls, or anything else. So if you have ugly arms, you'll always have ugly arms. Interestingly enough with regard to this discussion, everyone in JS's family has really great bicep genetics (not that JS cares). Again, everyone is way too concerned with their bicep routines. My little brother is absolutely floored by how much bigger his arms have gotten...and we do minimal curling. His arms have grown a lot because we squat like crazy, and train our back 4 times a week.