bigger rear disc in rear than front a problem I assume?

I agree that the lever arm of a larger rotor does have an impact but if a tiny caliper is pinching a huge rotor, I don’t see the rotor size slowing the wheel any faster. A tweeker with a 3 foot breaker bar won’t crack a nut loose quicker than I could with a standard ratchet.
There is a principal known as “the point of diminishing returns”.