It doesn't seem right but here goes:
Fluid does not go through a smaller opening like toothpaste squeezed from a tube. What it does is convert some of the fluid's pressure into kinetic energy, so the same amount of liquid flows through the opening, at a much faster linear velocity. Then the additional kinetic energy is converted back into pressure on the other side of the hole. Some of the energy is lost in the two conversion processes.
What the pump sees is a mathematical addition of all the flow resistances in the system. At the desired flow rate, each foot of pipe will have a pressure drop. things like valves, elbows, etc. are usually added in as an equivalent number of feet of total pipe length. Then you can calculate the total losses in the system.
The fitting in question will probably add no more than 1/4 psi to the losses in that tubing run.
There's only one time I would take the time to enlarge the fitting and that's if it was on the inlet size of the pump. One should try to eliminate all restrictions from the inlet size. That's why there should never be a filter on the inlet of any pump, just a screen to keep out the larger particles.
R.
Energy is never lost. It is simply converted into another form, usually heat.
What you are missing is the pump is stupid. It will keep hammering away and increase pressure as resistance is found. It will draw all the amperage allowed trying to keep the impeller speed up. That leads to very high pressures, cavitation, and higher fuel temperatures.
I, too, have had a return line that was too small. It resulted in uncontrollable fuel pressure. The regulator couldn't bypass enough fuel due to line restriction.
Replacing the small diameter fuel line with a larger piece cured the problem.
