Decreasing the size of the grounding and/or grounded conductors.
This is a big pressure washer
My guess is a Grounded, or Neutral conductor will not be used for this motor, unless there is some type of motor controll, control panel display and lighitng incorporated in the pressure washer.
If this pressure washer utilizes an adjustable speed motor drive, it may produce harmonic currents in the system Neuteral conductor, which will require you to oversize the Neutral.
I'm probably overthinking this as the motor probably does not even require a Neutral.
If you are refering to the EGC then hurk27 is correct and a #10 copper conductor is required per table 250.122
Michael H.