Have you never heard of a thing called 'braking'?
True as far as your statement goes, but.
The leverage from a force being applied by a 20" lever to the frame, or from The frame to the ground, will be different than for a 18" lever. Very different.
On the front forks in particular, the distance between brake pads is consequently greater and wheel centre will transmit more torque back into the headstock, and hence frame, under heavy braking.
And this pedalling whereof you speak, our imaginary rider is now transmitting the forces through the wrong end of a 2" longer lever.
And so on and so forth.
And it's good that you mention pedalling forces. A standard flame already flops and twists like a MP being caught in a lie when my 1/8th of a tonne lays down the power through the pedals hard, and that's why I do not buy one (that and its a little cramped, but I could probably with the sizing alone.) This is why I am hoping that Brompton will see that a larger wheeled bicycle - not just a conversion of an existing frame - might attract larger riders, so might stiffen the frame to suit.
I do not know that they will go that route, but that is the hope. I don't think that basic premise is at all difficult to u derstand?