Originally Posted by hokiefyd

An interesting twist to this conversation is the old SunTour XC derailleur:

SunTour XC derailleur (3 pulley system)

The "length" of the cage is equivalent to a short cage (draw a line between the upper and lower pulley), in terms of how the lower pulley would rotate forward and rearward during shifts. But it had larger wrap capacity because of a third pulley. I reckon that design had enough downsides (complexity, friction, cost, etc.) that it was dropped without further refinement.