Lofman's got it on all accounts: they need to have 2 of fewer provinces and you need to occupy them all (i.e. none held by allies). You can change this by opening defines.txt and looking for the entry for min. annex size. Be aware that, from my experience, setting this too high will make the AI go crazy and try to annex everyone in one hit, and they will often refuse to sign a peace if one of their allies takes a province when they want to conquer a whole country (I recommend setting this no higher than 6).
You'll have to settle for a handful of provinces at a time, so just take whatever you can and ensure that you can hit them hard and fast once the 5-year truce has worn off. I find it's usually best to try and encourage civil wars and rebel governors and conquer those, rather than hitting the target country while it's at full strength.