You don't have to be a physicist for this. PSI is simply the amount of pressure/force being exerted per square inch of tube/tire. While the actual amount of air you're putting in a larger tire may be more in terms of total volume, the fact that it's going into a larger area means the force being exerted on a square inch is actually less. That translates into a more comfortable ride. As tire size increases, the PSI is always going to decrease, even though the total volume of air is more.

But comfort on a bike also depends on your weight and other factors. Lots of folks can run standard 23 mm tires without putting 120 PSI in them and not pinch flat whereas heavier folks can't, which is why some look to 25 mm tires, tubeless and other things for a bit more comfort and protection.