The brass is not necessarily stronger....
I wouldn't know that.
I have no scientific proof of my opinion. I have only the musings of one who has a slightly more than casual familiarity with forces and materials.
Here is my logic:
In the case of the Remington, the forces that are applied as the ball travels down the barrel push forward on the barrel with a force that is roughly (and maybe exactly) equal to the force applied to the recoil shield. The force stresses the tensile strength of the frame top and bottom as it tries to stretch the frame. So you have force trying to stretch the brass at the top of the frame over the cylinder and the bottom of the frame just above the trigger. Hawg's photo is clear evidence of the force applied to the recoil shield. (Though I truly did not realize that 25 grains would do that after 18 shots)
In a Colt open top revolver, the only member of the revolver upon which this stress can be exerted is the arbor. And more correctly, the mating between the arbor and the frame. So we have a steel rod that is threaded into a brass hole. And all of the force pulls on that rod.
It is the same or (I think) nearly the same during loading when one loads with the cylinder in the revolver
I imagine, it would not be hard to determine to tensile strength of the cross-sectional area of brass we are taking about and compare it to the sheer strength of the threads in brass.
Machinery's Handbook probably has that data. I have never compared it or researched it. To me it seems intuitively obvious, but there are plenty of things that are obvious to me but are still quite wrong.
I call upon someone to describe the forces in a way that is different from my understanding. I also call upon someone to dispute my opinion about the relative strength of open top and full frame revolvers.
I am not saying I am right. I am saying I think I am right.
My reading of history convinces me that most bad government results from too much government. Thomas Jefferson