It's also size of your hand. Grip.
For power and distance, I think that technique counts more than pure-muscle arm strength, or, rather, that what we see as arm strength is long arm, large hands, and looseness, or snap, at each of the joints involved in throwing. The arm is used like a whip -- shoulder, elbow, wrist, and fingers -- each joint adding acceleration to the ball.
My hunch is that the best QB's are guys who can see and "think" and throw a bit quicker than the game that's being played around them. They can see a defense and sense that a receiver is about to come open without going through a "top of the brain" reasoning process. A shortcut from their eyes through their brain to their arm, saying, "that receiver now".
They have a shortcut throwing motion so they release the ball quicker, rather than having to wind up.
For an example, in the early '60s the Redskins had a big QB named Norm Snead, who could throw rockets. One season, the Skins had no offense except Snead throwing 60 or 70 or 90 yards in the air to Bobby Mitchell.
A couple of years later, they traded Snead to the Eagles for Sonny Jurgenson, who could not throw quite as far (almost, though), but who got rid of the ball quicker and more accurately.
In his late '30s, people began to notice that Sonny no longer had the "arm strength" that he had had. He quit throwing the ball 70 yards downfield. It didn't matter. Teams were shifting to the "modern" zone defense, and Sonny could spot the seams and hit a receiver before the defenders could seal up their coverage. It was almost as if they evolved the defenses right into Sonny's strengths, and he never lost his sight, sense, and quick release.
(For the "late model Sonny", look at the recent biography of Vince Lombardi, who, by the way, said something like, "If we had had Sonny with the Packers, we would never have lost a game". Both Lombardi and Jurgenson attended the SB after 1969, Lombardi's season with the Redskins. I think it was SB 4, Vikings and their terrifying "purple people-eaters" defense against Kansas City. Early in the game, both Lombardi and Jurgenson were looking hard at the defense. Lombardi turned to Sonny, who was sitting a few seats away...nodded towad the defense, and Sonny grinned, and signalled, "I see it, too. We'll own that defense".)