What I've seen repeatedly is injuries caused by switching suddenly from a 5 or six wt on brooks etc to intensive casting with a 12 wt. It's worse with age, obviously, but even young, strong, excellent casters can get very sore.
Case in point is one of nephews who's been fly fishing since 10 or 11 yrs old and working at a remote fly only fishing camp (trout) from age 15 through college graduation. He is about 235 and was in college on a football scholarship. He was easily casting bass poppers with an 8 wt 80 feet at age 14.
He came down to help me paint and catch some tarpon. The day following the first grueling 4 hour session of blind casting to locate fish, I caught him painting left-handed and started grinning. I'd seen him unconsciously massaging his right forearm earlier that morning and, of course, rode him a little about it.
It wasn't from the fish he caught either because he reels right handed.
The backcast muscle and the elbow are the most common prolems I see. None of my friends will do as I suggest (3 minuted of isometrics twice a day right before eating, for a month before coming down here), and all of them wind up with burning forearms at best.
It's like any physical endeavor - if you don't exercise the muscles you need, expect problems when you "overdo" it.
As to casting big poppers or Dahlberg divers etc with a 6 wt, yes it's a lot more work than with an 8 wt. But casting even small flies in the intermidible wind with a 12 wt is also a lot of work.