The second part of this puzzle that makes things even more puzzling...
Is the fact that when you look at longer distance runners...
Their legs, while not as impressive as the sprinters, still look pretty damn good overall.
But their arms on the other hand, are surprsingly skinny in comparison.
Now here's the explanation for this apparent inconsistency:
You see, whenever you run, at low speeds or high speeds...
The motion of your legs creates a rotational force that attempts to pull your torso from side to side with each stride.
However, in order to keep your torso remaining stable and forward-facing...
Some part of your upper body needs to generate an equally strong rotational force in the opposite direction...
To neutralize the force from the legs.
At jogging speeds, this is accomplished almost entirely by your core...
As the oblique abs stretch and contract in opposition to the legs.
Which is why when you see joggers run, the arms seem to do as little work as possible.
And with such a minimal amount of effort, it's no surprise they don't grow.