OK I guess this is how the 720 is. Maybe because the pump is in the rear?
Trying to form a reason for a larger and smaller line and why?
The smaller the line the more pressure drop over the length, on the return you really aren't concerned about pressure drop, but on the supply side you are...
It's like ohms per inch in electrical, the more resistance total, the more voltage drop... You use a bigger wire to carry more current with less voltage drop. In fluids, rate of flow is current, pressure is voltage. restriction to flow is resistance.
And, you supply more than you return, because you're burning it... So you'll always be returning less than you are supplying.
And the bottom line is cost, so they'll use the smallest line they can get away with.