The long head of the triceps muscle tends to get under worked and therefore is generally underdeveloped. In order to truly give your arms that size you are looking for you need to develop this aspect of your triceps. Try shifting your focus to overhead and behind the head movements to increase the mass of your long head.
Overhead rope extensions ( Checkout my previous post on how to execute this exercise ) http://h4training.blogspot.com/2009/12/eom-overhead-tricep-rope-extension.html 1 x 10 reps
Lying behind the head barbell extensions 1x12 reps
Rest 90 seconds and repeat 3-4 more times.