Definitely genetics. You picked two bodybuilders that have some of the best arms in the sport. Phil Heath is another one that has some of the sickest forearms in the history of bodybuilding..... Well he's the owner of the best arms in body building in my opinion.
Frank McGrath has always had humongous forearms that are ridiculously vascular as well.
Ronnie didn't have Brazzers
Ronnie didn't have Brazzers
Jon Delarosa has them also. Genetics like Lee Priest had.
Big forearms is very chill looking
I'm always surprised to see training routines that don't involved directly hitting them. Everyone uses wrist wraps for back, etc. too