That’s usually a reference to a specific suit of armor in the Tower, being somewhat incorrectly generalized. Rigid plate armors have to be pretty well fitted or they will hurt you, and some fighters (John of Gaunt comes to mind) had much heavier shields than swords, so their shield arm was likely bigger.
Personally, after about 20 years of combat sports and martial arts, I developed disproportionately large triceps in both arms. Biceps are pulling, lifting muscle, very useful to weightlifters and stevedores, while triceps are striking and hammering muscles, useful for anyone delivering force. As I’ve aged and had less time for vigorous combat, this hypertrophy has declined.
EDIT: Oh, and I’ve always used my fists for pushups, opening doors, minor demolition work and whatnot* so I have the characteristic bone deformations that produces.
* whatnot specifically includes makiwara boards 
