You only need the rectangular pump if the carburetor isn't huge.
Correct, they go by different liter per hour.
Small carb, small motor less fuel needed
I run fuel pumps on all my motors. Going by gravity also works sometimes, but for a constant consistent flow of fuel the fuel pump is best. By gravity you have the risk of stops, go, turns, and bumps effecting it. Plus with a fuel pump the fuel pump only pumps out how ever much your pulse fitting is pumping. Meaning if your motor is at 1500 RPM, you will get 1500 pulse pumps, and it raises as your RPM raises. Unlike a 2 stroke that wants fuel every down stroke, it doesn't need a fuel pump.