What is the phenomena behind this DC generator plot?
At low load (low used power) the efficiency is low because the power is at the same order of mechanical losses like the friction losses and losses of coupling the motor to the alternator and losses at the alternator like field, hysteresis and eddy current losses.
However when the load increases these losses will become low compared to the load power.
When reaching full load the generator becomes overheated which decreases the efficiency and also the counter torque coming from the load (magnetic) will increase to limit the capacity of the motor to deliver the needed energy.