You do know that an arm is a three dimensional object with no sides, it has length, width and depth.- Good try - but your theory does work on 2 dimensional cartoons.
True, but let's be real...
So top to bottom is 7... So that's 14.
Let's say that his width is 1/2 his height.
That's 3 + 3... 6
So that's 20... so let's add in a 1/2 an inch of play top and bottom and you still get 21.
That is probably relatively close to accurate.
There's no way that there's an extra 5-6 inches floating around unseen.