A regular coffee is "single cream - single sugar". It is not a size; it is a way of taking coffee.
Timothy's World Coffee offers coffee in five sizes - small, regular, medium, large, and extra large. What is the difference between a regular and a medium-sized coffee? Why is "regular" smaller than "medium" but bigger than "small"? I can have a regular medium or a regular large or a regular small, but what's a regular regular?
I blame McDonald's. I think they're the culprits who introduced "regular" as a size. In the McDonald's universe, regular = medium.
But, then, why is regular coffee in the Timothy's universe a different size than medium coffee?