Just to be clear, you're not forced to ransom anyone. Imprisoned vassals can't join factions, so keeping them in jail makes them unable to act in any significant way against you, and allows you to get stronger.
Generally the only times it's worth it to ransom a vassal are when you really need the money, when they don't have a big opinion malus and when they're old, preferably more than one of these conditions being met at once.
I was also going to say that how title revocation works is that you can either take away either one or two of their biggest titles (if they have more than one, obviously) or just two small ones, for example:
Taking 2 counties from a duke or taking their single duchy.
Taking 2 duchies from a great duke or taking two counties.
Taking 2 duchies/counties from a vassal king or taking their only kingdom.
Taking 2 kingdoms from a great king or taking 2 counties/duchies
I really thought this was how it worked, but the wiki apparently says something totally different (pic related).
So that comes back to alternative 2, which is generally my preffered approach anyways due to most of the time being easier to manage than fighting against a large rebellion.
Regardless, be aware that often it can actually be better to take a county than a duchy from a duke. Power comes from land, and taking their demesne away is often much more effective than taking away however many troops and tax they were getting from their vassals.>>261300
Have you never had a realm larger than ireland or a medium kingdom? Big realms get a lot harder to manage, sometimes you conquer a bit more land than you should and suddenly have a lot of angry vassals looking at you. Or alternatively, you just have a really hateable ruler, and those 130k vassal troops rebelling will easily crush your "massive" 20k.