The measurement is taken 2 inches from the butt end. Your standard grip is likely a .580 or .600. If your shaft diameter is actually .620, then your grip is .40 or .20 oversized. That doesn’t take into account any extra wraps of tape. Some people build up the bottom hand, which is why companies started making the “+4” grips. Those are grips with the bottom hand of the grip the same size as a regular grip with 4 layers of build up tape.