I don't know the science exactly but the bore ought to be bigger on the big 'uns and smaller on the small ones to be efficient. I suppose you could sleeve the cylinder and work it like that but the smallest one would be too big ....
Probably works easiest with the sliding-chamber fixed barrel system. But even then, you end up with overweight guns. Even the HW77 25mm could have been made 7/8ths size and not lose performance.
I like the HW range, they have a rifle for every circumstance!
Shed Tuner is the man to answer this one, he knows about esoteric things like 22mm pistons and that....