Bo,
The FET's inside the ESC are rapidly switched ON & OFF as comanded by the throttle, at Full throttle the FET's are fully ON and in this sate have the lowest internal resistance, at part throttle the FET's are ON & OFF, the transition between ON & OFF is not instantaneous and the internal resistance of the FET is much higher during these transitions, so more heat is generated.
It is counter-intuitive but in general the ESC is more stressed at part throttle than full throttle.
Simple empirical tests (2012 post on RCG,) have been performed at full and partial throttle, with the current measured by a wattmeter at part throttle being half that of full throttle yet the ESC (shielded from prop cooling effects) temp rise after 1 min was around 15°C at full throttle and 45°C at part throttle !
Steve