It's not going to do anything to a diff to always have one tire on one side bigger than another. Nor to a WWD center diff for the rear tires to turn less than the fronts (which they always do, in fact, because rears always follow a tighter turn than fronts) -- what you don't want to do is drive a 4WD with the center diff locked, or in some 4WDs (like my 460 Gelaendewagen) without any center diff, on dry pavement.

Rather than a strictly mechanical thing about the differential, it's a concern of the pumps and other controls that diverts torque to the differentials. The reason for mismatching concern is for there to be a continuous signal of different speed which is going to have the AWD system continually thinking that there is wheelspin (or slippage) and trying to divert power (as it compensates for this alleged slippage).

By the way, you're 100% about this sometimes being a problem with 2WD cars. About ten years ago, Mercedes sedans with a certain antiwheelspin software would, when faced with mismatched tires, put the engine in low power mode (to lessen torque output) and the car would suddenly just go slower, and slower, and eventually just barely crawl down the road regardless of how the driver stomped on the gas pedal!