The bore/stroke, the springs strength, the pistons weight, transfer port, dead airspace, barrel diameter and length, and the pellets weight/friction/stiffness are all factors. The nitro in a Crosman is imo too strong, meaning it's more than the gun can use so it doesn't make use of all that extra spring strength. Like a car that just starts to spin the tires at 400hp, so going to 600hp isn't going to help.
If the coil version compresses the air all the way, then compressing it all the way but faster like the nitro does the same thing but is harder on the gun. Or fi grease slows it it's still as fast as or faster than the coil so no real felt loss.
Now make the chamber larger and it has more air to compress and that will make more power.
If the pressure in the nitro has too much power, say 500psi too much, then a 2500 dropped to 2200 from cold is still too much so the new power out the barrel will be the same. I'm sure it isn't really the same, but increasing the pressure wont give you the returns you want. Say a coil spring barely making peak power in a Crosman, then drop spring strength in half you'll lose half the muzzle energy. Since the oem spring has nearly maxed out the power of the gun at 18 it means you could double the spring pressure but probably only make 19-20. Doubling again probably won't add anything at all, but what will happen is those stronger spring will be more than twice as hard on the gun/scope. This is why magnum guns don't simply have stronger spring, they have to add bore and/or stroke to get it. With more volume they can now take advantage of a stronger spring.
The the crosman peaking at say 18flbs, with a spring capable of making 22 in a larger gun, isn't going to do much better than 18 in the gun. It does however allow the spring to have hangups like grease and cold because those reducing the spring by 10% it still makes more than the gun can make use of. Now a coil barely strong enough to make 18 under ideal conditions gets cut 10% by cold sticky grease will drop the guns power 10% to match.
There are many variables, but that's the gist of it.
So I tune coil guns to get the best power, and tune nitro guns to have less excess scope abuse. They get more power too because the tuning bumps the potential of the gun itself up, but both that bump and proper tuning reduces the shock of the stronger nitro to save the scope. It also helps accuracy in both guns.
And yes, my HW90 had an optional pump to adjust the pressure, but I never bought one. People claimed they could drop the pressure a bit and see almost no change in power, because it was overcharged. Apparently many dropped the pressure quite a but, like 30% but the power drop was nowhere near that. The power was still good but it was much easier to cock and a much nicer shooter. As mentioned with a Crosman coil gun if you dropped spring pressure 30% you'd lose 30% power because the spring was not stronger than the gun could use. Make sense?
So imo the Crosman should have the same nitro, same stroke, but a 3-4mm larger bore and it would be maybe a 22-24 ftlb gun. No extra cocking effort, plus it would be quieter, smoother, and not break scopes. But I don't work for Crosman so it probably won't happen.