Sanctify7,
Are you asking me to be succinct? That is somewhere between impossible and torture
There are a number of overlapping factors that predict which caliber would be quieter. The same factor does not always dominate. An attempt to oversimply with blanket statements of an absolute nature are not helpful.
As an introduction to the answer to your question; you do not need to drive a .30 as hard to make a given FPE as you do to get that from a .25 or .22. This is because the larger base area of the projectile delivers more force at a given air pressure - assuming you can flow enough air. "Driving hard" has to do with the percentage of maximum power that platform could deliver in that caliber, and with that projectile weight. Part of the equation is about how wide the flow area is, and how long the valve is wide open.
If you run any PCP close to its maximum power it will become louder. With a .30 you do not need to run it very hard to make significant power. But, if you run a .30 as hard as you need to run your typical .25 or .22, then the .30 will make much more power
and more noise.
If your regulator setting or air tank pressure is low (1000 PSI or less), and you insist on making lots of power, then the only way to achieve that, is for the valve to stay open much longer. This will result in the muzzle pressure being higher, which will make more noise.
I think I am already repeating myself. I desperately want to draw charts of air pressure decay curves inside PCP barrels. Rather than do that, I suggest you read the threads below. They cover this topic clearly, with plenty of graphs; with calculated or measured data - rather than blanket statement about complex conditional interactions:
Maximizing power for a minimal noise level:
Maximizing power for a minimal noise level, Air guns, Airgun Forum
www.gatewaytoairguns.org
Acceleration and velocity of a pellet inside a PCP: Acceleration and velocity of a pellet inside a PCP - internal ballistics, Air guns, Airgun Forum
www.gatewaytoairguns.org
EDIT: One more thing that is often not considered; the larger and heavier the projectile, and the more energy it carries, the more noise it makes when it hits the target - or whatever is behind the target. That could be a reason not to plink or pest with a .30 in urban areas. But, it makes sense for larger critters, or longer range shooting - assuming projectiles with a better BC than the smaller calibers. The latter is not a given: .22 slugs tend to have a higher BC than .30 pellets, for example.