There's also the question of profile, many different tires have different profiles and behave differently at varying pressures.
For road with a hardtail, I run a big 1.75 back tyre with a pointed parabolic profile. Deeper grooves in a thicker tire help in the wet, and they go to 100psi and don't lose much air. They also provide a bit more cushion than the narrow tires, and don't snakebite so easily, so with no fork to soak up a big bump a fatter tire is more durable, and generally hitting stuff if you really mess it up it's the front wheel that takes it, but it's routine that you hit something with the back wheel and all your weight on that axle, with no suspension it's much tougher with a big rear tire, but counter-intuitively it's a harder ride, there's more pressurised air in the tire so it's more bouncy, like a hard rubber ball, less damping from the tire wall changing shape. The contact patch is visible in the dry as a lighter dust mark, and it's actually slightly narrower than the contact patch on the front, 1.2, which has a flatter circular profile and a good 180mm of travel in the forks.
I have been told by pro riders that this is a bit wrong for a mountain bike, but it works pretty well for me on the current road setup. Matching tires makes for the best handling, so I will probably just put a big one on the front matching the rear one on next.