Lu's answer:
"The mast length on the Robbie like that on the 206 has to be long because of the low rotor blade/rotorhead interlock. It supplies a longer moment arm."
Means:
Long mast required to accomodate CoG range, since 'teetering' rotor heads supply no direct attitude changing moment - the CoG will hang directly under the center of the rotor head in a hover - to keep the angle between mast axis and disc axis small you need a long mast.
(any heads with flapping hinges (or virtual hinges) at a distance make a restoring force which resists attitude differences between helicopter and disc - (less g dependant for attitude control authority) - so they tend to have much shorter masts and have relatively more CoG range)
- I hope I haven't made it worse.