You ignore the fact that a larger realms has to run a lower tax rate, which pretty much eats away the gold benefit.
That's a pretty big assumption. I wouldn't think that the tax rate decrease required by taking an additional region would come close to offsetting the amount of gold you gain from taking it.
In fact, I'm nearly positive (haven't run the numbers, but don't feel like it's necessary) that under either the old or the new system, it wouldn't come close at all.
