A hydraulic system is a closed loop system - and quite often you will find strainers on the suction side and filters on the feed side - look at enough heavy equipment and you will see that you can design without compromising for flow. - A fuel system is not a closed loop system. Running a fuel filter before the pump inlet saves the pump from the junk that gets introduced in the fuel and the ever decomposing foam from cell - general rule of the inlet size should be larger than the output - I generally run a 100 micron on the inlet side of the the pump and no filter on the output - one 12AN line with filter in and and 1- 10AN out. Every car made today has a filter of some type in the tank on the suction side as a pre-filter.