Doug, I don't know about everyone else but I kind of like the slightly smaller extra tires idea. The part of the extra tires farther away from the mount have greater leverage and wear the front wheel bearings more. If the tires were a tad smaler when on good solid surface you would not have the extra force, just when really needed.
Don't overdo the size differential as when the tread of the smaller tire engages softer ground it will have a smaller forward velocity than the larger tire which will tend to make for some slippage.
As far as whether or not that will effect the likelihood of lifting off from a "Magic Conveyor Belt", I don't want to take a stand, just yet.
Pat