It's very much more complicated than cross-sectional area. For instance, tribology (the science of lubrication) treats rotating journal bearings as pumps. Peak pressures can reach thousands of psi at high speeds and loads. Look up the Reynolds Equation. What would the flow through the bearing be under those insane pressures? Clearances around the bearing vary a great deal with load, as does pressure. There are lots of SAE papers on journal bearing lubrication if you really want to go nuts.
Loads on the rods and mains can be measured in tons at higher rpm and throttle positions. To convince yourself, do a napkin calculation of how much oil film pressure you need to support even one ton of force on a rod journal. You'll find it's at least an order of magnitude higher than anything the pump can put out.
That's a good point, that the crank acts like a centrifugal pump and NPSHa (Net Positive Suction Head available) would also apply. Insufficient NPSHa can create a vacuum and/or cavitation, aerating the oil (in the case of a engine, threw the bearing and lifter clearance, above the crank) and one reason that inlets are always larger than output ports on most pumps.
Also, in a engine, if the crank is pulling oil at a higher rate than a pump can supply, the oil pressure should go down.
Anyway, no one has to modify their blocks, like I do but, it makes since to me and those are the reasons I do it.
