The build up of pressure under a helicopter in the hover is observable on a light wind day - the altimeter goes down as ground effect occurs, but only to the tune of about 30 ft (1mb). However the ground reducing the induced flow is the major effect. The increased pressure diverts the flow outwards (it has to go somewhere).
The benefits of low hover are a balance between the good news of the GE and the bad news of enhanced recirculation at the rotor tips. Hence, long rotor blades benefit most and wide fuselages reduce the leakage of the downwash up the centre of the rotor disc. This is why some helicopters have a demonstrably better "ground cushion" than others. The R22 is rather poor in this respect
(short blades and egg shaped fuselage).
Long grass slows the outflow and allows it to be picked up by the rotor tip vortices, which reduces the rotor thrust.