Best I remember, front drums were bigger because of weight bias to the front and weight transfer towards the front. Meaning the front drums did more work. More braking power on the rear would just tend to lock up the rear wheels.
that’s what the proportioning valve was for, to limit the pressure going to the back brakes. Or in the early cars, the secondary valve did that before the prop valve came into play. 9” and 10” brakes were never mixed, and are 2 completely different setups.
