The theory is easy to understand: perform John Gruber’s Mini-predicting math backwards. The iPad Mini uses iPhone 3GS-density screens at iPad resolution. What if an iPhone Plus used Retina iPad screens with iPhone 5 resolution, keeping the rest of the design sized like an iPhone 5?
The reasoning is sensical, but I still don’t like the idea of an enormous iPhone. Apple following the status quo instead of doing what’s best seems off to me.