If you do not want to give land at all because that would be bending over to demands, tell them your intentions then buy the meter and hit back with razes on their t/m's and ambushes on high off attackers, do so to mitigate land away from the provinces they want it on, and to destroy acres while at the same time dammage their military the most, use pitfalls, bribe generals and use smite to your advantage. If you are arround the same size for that other kingdom to start a conflict where you will dig in and hit back this way is hardly worth it.
You could also give the land to someone else who you would rather want to give it to.
But as many mentioned warring might be an option, there is some mid size kingdoms arround out there with 25 provinces that really only have the numerical advantage, but are not particulairily good and may lose to a 22 man kingdom at times.