The 12 Ga shell is a bigger base, which translates to a larger piston, which translates to more force trying to blow the action open.
Let's approximate:
12ga about .8" bore
Diameter=.8 radius= .4 area = pi*r*r = 3.14*.4*.4 = .5024 square inch
410ga about .45" bore
Diameter=.45 radius= .225 area = pi*r*r = 3.14*.225*.225 = .1590 square inch
From your data 410ga pressure is 12,500 PSI (pounds per square inch)
.1590 square inch times 12,500 PSI = 1,987.5 pounds of force.
From your data 12ga pressure is 11,500 PSI (pounds per square inch)
.5024 square inch times 11,500 PSI = 5,777.6 pounds of force.
Other forces are at work, for instance:the same pressure that is pushing the piston back is also holding the case against the cylinder wall. The point is that the Maximum Average Pressure (MAP) is not comparing apples to apples.
For what it's worth:
500S&W .5" bore
Diameter=.5 radius= .25 area = pi*r*r = 3.14*.25*.25 = .1963 square inch
500S&W pressure is 60,000 PSI (pounds per square inch)
.1963 square inch times 60,000 PSI = 11,778 pounds of force.