I think part of the key here is that the Proof House Proof Tests. So they do not merely pass the gun because it has 20 thou, they shoot it with a proof load and see if there is enough THOU to avoid POW.
And, it does seem like I have seen more pressure problems with small gauge guns than with larger bores. Don't remember enough physics to recall if a cylinder of a larger diameter would be stronger than a cylinder of a smaller diameter for a given pressure.