One of the reasons, aside from smaller size/weight, the downwash should be less is that at this size they don't have to worry about required safety clearance between the rotor arc and the island of the Marines' air capable ships. On the V-22, the rotor disc is smaller than optimum for that reason and so that an Osprey can fold up within its own length and width of the empennage.
Regarding the HH-47, now there's downwash! The UK found as their Chinooks approached the LZ in troop insertion operations, the downwash would actually set off shallowly buried land mines.