Is it my imagination, or is drumming helping me to develop musculature in my arms such as I have never experienced before in my life (but only wished I could have)?  

Suddenly, as I'm drumming more and more lately and since I'm playing heavier music than before, I seem to be developing muscles in my arms, and the ONLY time that this happened for me to ANY degree at all was when I was working out in a gym 5 days/week.  But the gym was boring, while drumming is always fun.  And it honestly seems as if my arms are developing even MORE now than when I was at the gym.  Well, and my legs too, since I'm using double bass more than ever.      

Anybody else experienced this?  

If you use fingers especially, your forearms can definitely develop as they are all connected.

Like Popeye!