This is definitely an interesting suggestion. There have been a number of ideas thrown out in the <a href="
http://triumph.toomuchstupid.com/board/t_t5">Cash Flow</a> thread, and I think this and another in there stand out as good ones to me right now.
1: Make the resource mine level (currently 0.0015) less as a country grows to be very large. The reason I like this idea is that, realistically speaking, it's going to be more difficult to collect resources efficiently from a huge piece of land. The percentage couldn't be <i>too</i> small, though, because a large country also has much more land to protect. It's an advantage fighting one on one, but it's difficult to maintain a fight on multiple fronts. I also don't want it to be too much of a punishment to expand, because that's an important part of the game.
2: Subtract a "maintenance" fee each cycle for each unit the country owns. (This would, of course, also require me adding the ability to sell units.) This is also completely realistic.
What do you guys think? Both are actually fairly easy to implement, so perhaps we can work to improve the balance and realism.