Ray : I would think you would want both rear tires as close to the same circumfrence as possible . Though you could play around with air pressures to make up slight differences . I talked to a guy a few weeks ago at Go Pro in NC. He was insisting that if a Sprint track as example has 10 turns 7 left 3 right you ALWAYS put the larger tire on the right side . I tried to get him to explain his rationale of thinking and I was NOT buying his answers .
To my way of thinking you would be upsetting the Balance of the kart , Perhaps even making it HARD to lift the inside rear tire on cornering . Maybe even affecting Castor Camber ?