I'm pretty sure it has to do with pressure loss the larger return line allows easier flow with less pressure. there is a pressure drop through the heater core
Yup. The heater core outlet is often larger than the inlet. That way the outlet restriction is less than the inlet restriction, minimizing dynamic pressure in the heater core. There's often a restriction in the heater inlet plumbing--right at the engine on certain Olds and Chevies. The inlet plumbing is restricted down to about a 1/4" diameter so that the heater core can't be over-pressurized by the water pump at high RPM.
