That's actually not correct. All Shimano 4-pot brakes use the same size pistons. As per the above post, Saint, Zee & M8020 XT are the same caliper, just with different printing & different factory-fit pad types.
The MT501 (and MT500 2-pot) lever is essentially a relabelled M6000, which shares its basic design with M9000/M8000/M7000, but minor detail differences; no contact stroke adjust and no onboard knob for the reach adjust; it needs an allen key to tweak. The 520 caliper, like the M6000 takes its basic design from the better models, but uses a straight hose connection rather than the banjo of the others. Probably the biggest difference, and the one that will most (but still only slightly) affect power is that the M6000/MT520/MT500 use the standard BH59 hose, where SLX/Zee & above use the BH90 "high-power" hose. The thinner-walled standard hose can be slightly more prone to deformation under heavy braking, so will take a little bit of power out of the fluid displacement. The olives are the same for both hose types, so you can upgrade the hose to BH90 and achieve the same brake performance as the better spec levels.