Why greater rotor puts more stress on fork (no matter how and where the caliper is mounted) from physics point of view:
Friction depends on the materials touching and the force they are pushed together.
The materials depend on the quality of rotor/pads.
The force depends on how hard you can squeeze the brake lever and also on quality of the brake (caliper design, brake hose strength against stretch, brake fluid, etc.).
So given a brake and given how strong your fingers are the brake's maximum friction force it is able to generate is always the same.
Now - the further the braking point is from the center of rotation the more leverage there is and the greater braking torque can be achieved. Higher braking torque puts more stress on the fork.
The other thing and non-relevant is if you can use all that braking torque. That depends on the weight of the rider, the tire (contact patch and tire tread), and the ground (asphalt, gravel, mud, ice,..).