i also agree with caba.
i like what they did with garrisons. good design, and i dont really think it needs to be re-evaluated. it works fine.
consider it this way: you want to siege a town with your ~500 army (more, or less, if youre king/marshall/whatever), and its garrison is lets say ~3k. well, get your troops ready and make an attack, try to make as many casualties as possible, and when the tide starts to turn and your side is either running out of ammo or starting to take losses, retreat. keep doing that, keep harassing, attacking, pushing on penalties, and eventually youll take the city.
this is realistic, its how a force would be required to take a city, and i like it.
taking a city or an important castle with a single battle that ends quite quickly is imo just ruining for gameplay and immersion.
the one single bad factor is that lets say you make an attack, and need to go reinforce your army due to losses - unfortunately there is no way to have a reinforcement troop come to you, or for you to leave someone in charge to keep the siege up while you go back to a town/castle and get more troops.
if this was somehow possible it would make the system perfect.
in fact, perhaps the easiest fix, is simply to add an extra menu option when sieging....something like "call for reinforcements", which would open another dialogue asking from which city/castle do you want to take reinforcements from. then it would give you basically the "manage garrisson" window where you could add troops from the garrison over to your army. once youve done this, it would send a messenger NPC (the same as the game always uses for sending messages to lords, etc) to the city/castle you chose, it would create a small army of the player's faction of the troops you chose and it would move to your location and interact with you, or just wait there "accompanying" you.
im sure you might have to have some placeholder NPC like a recruiter as the first slot, so that when he arrives at your position and you take the troops you can take them all - first slot iirc is unable to be taken when taking troops from an army/patrol that you own?
im not sure of any specific issues that ingame mechanics and scripting would cause issue with this, but from what little i know of how the game mechanics and scripting/etc works it should work.
something like this would definitely make sieging a big town or castle more of an "event"...as it should be. and doable by the player him/herself, which it also should be (considering how useless your lords are most often, especially if you arent the king/marshall - but even so quite useless if you are).
but on the OP topic, i see usual garrisons in towns of 2500+, have seen up to 8000+ more than once. i tend to play with more troops than less. i give the AI lords bonus to their troop sizes and such, usually faction lords are running around with 800-1000 size armies, compared to a normal size army of myself at around 600-700 (when im king, hold a handful of castles, have at least 1k+ renown, and am marshall). so its only natural that ai will be able to garrison alot of troops. and i like it.
maybe i shouldve broken this up and put it in suggestions...sorry about that

, i just starting typing, and it sorta came to me on the fly...