No increase in volume - they have the same diameter and thickness rotors.
The 48 has better rotors and provides more volume. But don't use the 48 diesel pump housing, use the gas 48 housing in a gas application. The 48 has a rotor .100" larger in diameter so you have to machine the pocket diameter to use those rotors unless you score a Viper Truck gas engine 48 pump housing.

And......what do you hope to gain by providing more volume?