To answer your question why a larger tire needs a lower pressure. Regardless of tire size, your Jeep requires a specific amount of air in the tires to support the weight of the vehicle. The amount of air required doesn't change but the volume of the tire carcass does. A larger vessel of air will be as a lower pressure than a smaller vessel containing the same amount of air. The tire itself supports a range of pressures because it supports a range of vehicle weights.