As Shreky says, genetics play a huge part.....but do movements that focus on the long head rather than both or the short head as that is what will build the peak
Narrow grip bb work
Focus on the contraction, if using db's, rotate the pinky as far outwards as you can at the top and squeeze the muscle for a second or so.
Incline curls rather than standing or a vertical bench.
Don't just throw the weights around, if you want to develop a small muscle in a particular way, you really need to drop the weight and focus on the movement