How can an all flying rudder be almost 100% more efficient than a vertical stabilator and rudder? How can an all flying rudder be almost 100% more efficient than a vertical tail and rudder?

According to the Zenith school of aircraft design, a rule of thumb for the size of an all flying rudder is 7% of wing area. For a classic vertical tail and rudder, the rule is 12%.  That's an increase in area of almost 100%!!

A classic rudder foil, something like a NACA0012, has a max cL of roughly 1.5.

A vertical stabilator and rudder is like a wing and an unslotted flap, which adds cL of about 0.7. That's a total cL of about 2.2, which works out to an increase in efficiency of 50%.

So... why is an all flying rudder 100% more efficient, if a vertical stab and rudder produce 50% more lift?
Because on the Zenith the massively slab sided fuselage is the vertical stabilizer, and the rudder is still the rudder.  The surface area of the all flying rudder is about the same as the rudder on a normal plane, and it depends on the fuselage for weathervaning tendency.   There's no free lunch.  If you build a Zenith with a skinny boom tail and the all flying rudder, it will be quite marginally stable in yaw, like a Fokker DR-1.