Oh boy, it's late.....
I've noticed that doing alot of cuts with the bokken, (or bokken training in general such as kumitachi ), really build up the forearms.
Take a look at Chiba sensei or Shibata sensei's forearms; they're HUGE!!
What I've also noticed is that most advanced aikidoist also develope "hair cuffs" on the wrists. Don't worry ladies, I've only really seen it on the men.
I do agree on the "kokyu hands" also helping in the development of the forearms and wrists.