The nineteenth century is all about the balance of power. The three-province limit is debatable, but the great powers would have never allowed the outright annexation of a power of even middling rank in one go.
I think the game models this nicely in most cases. Countries like Switzerland or Denmark (or even Sweden or Sardinia) can be annexed in two wars. Real minors like Uruguay or Cube can be taken out in one go. Everything bigger is there to stay for decades, at least.
If you want to go on a WWII-style conquering spree take which provinces you can get from the US and humiliate them. Then break the peace treaty, reoccupy everything and repeat. You will lose a lot of prestige and rack up even more badboy, but you can do it, if that's your goal in the game. Just be prepared for the UK and Russia. They'll come knocking after uncivilized, warmongering huns. Oh, and it will turn the game into a wargame mostly, but that might be just what you're looking for.