I would venture to guess that cramming something larger than your elbow into your ear canal would be likely to have some serious medical consequences as well. (I'm not convinced that using something precisely elbow-sized is entirely safe either.)
That's always been the point of the expression, from my understanding. Nothing the size of your elbow will fit into your ear (without seriously screwing up your day). So if you are only allowed to stick things that size or larger in, then nothing should go in your ear (canal).
