Scaling down does not change the relative effects of the tail arm and the stab area on pitch stability. The formula for the tail volume, which measures the stabilizing power of the stab relative to the wing, is as follows:

TV = (stab area/wing area) x (tail arm/wing chord)

All the units, squared and linear, cancel out. The resulting TV can be used to calculate CG. For any given design, the formula gives exactly the same result for small planes as it does for large ones. I have used this formula on every plane I have built for over 20 years, from 2 oz CO2 powered models to .40 powered models and it works the same for both, as it should.

Increasing stab area allows one to move the CG back, and that can be very useful on scale models for two reasons:

1) Models land at much higher speeds relative to their size than full size planes and their wheels are smaller relative to the bumps on a typical grass field, and therefore models have a greater tendency to nose over on the ground. Moving the CG back decreases that tendency.

2) Scale models often have short noses, making them hard to balance. Moving the CG back obviously makes that easier.

I think that changes in stab area, within limits, are one of the least noticeable deviations from scale and also one of the most useful.

Jim