I think the rationale for Richard's prototype for height and arm length is pretty simple.
Picture a compass for measurement.
Now place that compass next to an object.
Picture the arc and trajectory of a football, and how it comes down towards the receiver. If you use that compass to measure the height and arm length of that trailing cornerback, the window and arc for the quarterback to fit his pass into becomes much smaller.
That's what I'm picturing anyway and I hope I didn't butcher the explanation!