Mike, it's natural that when your empire goes stronger further expansion becomes easier and easier. I don't like it too but there is a little you can do about it. I usually quit a campaign when I control about 1/3 of the map. IMO the most important thing that kills the balance is fleet. Most of the AIs (maybe except NAVAL EXPANSIONIST types) are unable to keep their trade lanes. They group their ships into useless fleets wasting 90% of the possible trade income. I think the idea to limit your own fleet is very good. You can even try to scrap your entire fleet. This way the danger of enemy invasion will grow along with your empire. You'll lose huge amount of your income too. It should rise the difficulty level significantly.