The thing is I understand physics very well and yes it is about the heat conversion, but I think what you are missing is that keeping the same amount of clamping force and pad surface area (basically saying that all things are equal) a larger rotor will create a larger torque force to stop the rotation. Even the equation to calculate brake torque requires the radius of caliper. On top of that a larger rotor is also moving faster through the pads thus creating more heat friction. I could link to many articles that explain it and I don't think I can find a single one that would argue that larger rotors don't allow for a larger brake torque. Also even though the rotor is the main heat dissipating feature in the system that is not it's main mechanical function. The reason good heat dissipation is needed is to keep from overheating all the components of the system like not warping the rotors and boiling the brake fluid.
Here's an article from an engineering company that says it very simplistically.
http://www.wcengineering.com/articles/brakes.html
and here's a white paper from a well know brake manufacturer with formulas.
https://www.apcautotech.com/getmedia/89aa6773-73d7-4f7f-b935-5e6bf2d28111/Centric_and_APC_Technical_Whitepaper_A1-The-Physics-of-Braking-Systems-8-2018_1.pdf
As for this having any bearing on the upgrade to the 19/20 brakes it's probably not a big factor since the new rotors are not much bigger than the stock 14-18 rotors. The big improvement is with a more even clamping force from the 4 piston calipers and an increased pad contact area. Possibly a better pad friction material and difference in over all piston area too, but I can only speculate about that.
I actually look forward to see what you have to say that could explain how rotor diameter has no effect on brake torque stopping power.