[framerguy]

With the drain pipe you are relying on gravity to push the water through. Much less velocity is generated (not sure if that's the right term). With the return you have something forcing the water through the pipe (the pump) so it can be smaller which created more pressure resulting in more flow velocity... Terry, help me out here! I'm right but don't know how to say it.
The drain line could in fact handle more GPH if it is forced through the pipe, but since gravity is providing the force (and thus the GPH you were told) the pipe needs to be bigger to provide the GPH you need.