I am not saying your wrong, but I kinda disagree. Everyone here is talking about stabilizers, and big compound lifts. Lately there has been a lot of articles on that.
But let me ask you this-
If you cut out all three of your deltoid heads, would the posterior deltoid displace even half-as much water as your front deltoid?
I am not smoking something, its just an example to measure volume. Honestly though.
Your wrists- can you reverse wrist curl nearly as much as you can regular wrist curl?
Can you row as much as you can bench?
Preacher curls require very little in the stabilizer department, and they would help anyones arms grow.
Everyone is concentrating on everything but the arms. Would you win a benching contest by squating 3/4 of a ton? Yes squats and deadlifts and everything else are important, but maybe, just maybe, his stabilizers are just fine, and his arms do need some work.