Example for the new system:
10 regions with 20 nobles: 100% coverage, 100 gold each region, 50 gold for each noble:1000 gold total for the realm
expand by 5 regions:15 regions with 20 nobles:
5 regions with 100% coverage, 100 gold each region, 50 gold for each noble:500 gold
10 regions with 50% coverage, 65 gold each (remember, additional penalties), 65 gold for each noble:650 gold
total for realm: 1150 gold
See? a 50% increase in regions should lead to a 15% increase in gold income. That number could be anywhere between 5% and 25%, of course, but it would be hard to find a scenario where expanding leads to less gold.
Actually, I can think of one--city/townsland/stronghold estate coverage. But you can probably free up one noble from each of those region types.
You ignore the fact that a larger realms has to run a lower tax rate, which pretty much eats away the gold benefit.
