Is it possible to limit vassal's armies in vanilla ck3?

I am the king of ireland, i have unified the country and made powerful alliances, but for some reason my earl and vassal cousin has an army with 300 more troops than mine, making him the dynasty head. i can't gain more troops myself as i'm broke and going through a plague, how do i kneecap his forces so i can access the dynasty tree again? honestly i have no idea why the game has it where you can have such a valuable, balanced system taken away by complete arbitration.