If the dyno Cylinders are lets say 17" in diamiter and you go on there in your car with 15" wheels - you'll get a power reading of XXX

Then you take the same car, on the same dyno, with the same setup, except change your wheels to 19", then you'll get a different power output reading - this is becuase the mechanical drag that the dyno uses to convert to tourq has been affected by the increased rolling diamiter of the wheels.