I have no idea how any of this works, but I recall reading that the ground effect threshold is some proportion of the aircraft's wingspan (half, I think?) so it

I have no idea how any of this works, but I recall reading that the ground effect threshold is some proportion of the aircraft's wingspan (half, I think?) so it could be that it's more difficult for a model just because it's smaller. (Maybe that was the wisdom behind making the spruce goose so big?)