I think a good way to limit expansion without hindering a player's strategic flexibility in determining how much to annex can be solved by a few things
(1) Every province you annex that is not the same culture as you will have triple the revolt risk.
(2) Each provinces (irregardless of culture) will give only a certain percentage of their tax value depending on the level of destruction that occurred during the occupation (looting level?) Over a period of time, say 30 years, this value will recover fully, however the player should be allowed to invest money in this province to recover the value more quickly.
(3) When a siege is won, the fort level should be diminished depending on the length of the siege. 1-3 months, no change, 4-8 momths - 1 level, 9-15 months - 2 levels, 16-24 months - 3 levels, and so on in whatever degree of time people feel is appropriate.
(4) It should cost a lot of money to garrison forts...i.e. in maintenance costs, and the garrison should be a slider, much like the army and navy sliders for maintenance costs.
(5) Non-annexed, controlled provinces should have a major revolt rate during the war. Maybe 25-50% depending on culture and CB shields and other things. This forces people to garrison conquests and not being able to blitzkrieg.
(7) A nationwide revolt risk of 1% should be imposed for every province annexed that is not of the same culture or religion as the mother country. This should come in the form of nationalism so that other effects can't neutralize it.
(9) During times of war, maintenance costs should be doubled or tripled. This should effectively make countries have short wars with limited objectives, like most wars of the time period, rather than vast periods of war with total conquest in mind.
(10) If a player is able to deal effectively with all these things and he still wants to annex, he should be able to, after all, everyone wants to conquer the world, and given these rules it would be fairly difficult to do so.
Finally, obviously these aren't perfect, and their are exceptions for every rule and point I've articulated, but anything beats a 1 province maximum annexation rate.
jbs