Ok...so why would a BODYBUILDER looking to BUILD THEIR BODY as optimally as possible AVOID training an entire muscle group directly?
Looking like these guys is more about NOT eating than building a lot of quality muscle. They are small and very lean. The ONLY reason they look big to you at all is because of their level of leanness. If you saw these same guys in clothes, you wouldn't notice them standing out much at all.
Here's what you do...go ahead and completely avoid training arms directly. Then, in 5 years after you finally realize they are lagging, then you can play catch up for the next decade.