TU Delft asks the question (section 4.5, ref f) :
Why is the YoYo ‘Traction Phase’ cycle time longer than expected, and the power output less than expected (soft kite 14m² -> 25m²).
The M600 has the same behaviour (ref video).
The ‘law of conservation of energy’ describes the interchange of Potential Energy (PE) and Kinetic Energy (KE). The kite starts at the top of the loop (circle or ‘figure 8’) with gravitational PE, and on descent it is converted into KE. The kite has to depower to avoid a tether break when the maximum tether tension is reached. Reducing the lift (or increasing the reel-out speed for YoYo), slows the kite down, and KE is lost. On the climb back to the top, there is not enough KE, so the kite slows further, with only the aerodynamic lift to do the work.
The cycle time is stretched out, restoring the PE, while the power output becomes more erratic (negative for the M600), decreasing the average power output over the cycle.
‘Scaling Up’ just makes things worse, more depower on descent, followed by a harder longer climb (with an exponentially increasing tether mass (kg)), see M30 -> M600 videos.
References :
f) Quasi-Steady Model of a Pumping Kite Power System, Rolf van der Vlugt et al (2018)