I looked at the files and the AI is rather simple if it comes to expansion. No losses means 200.

Go to Province
Is the province of an Ally -> Retreat
Is the province undefended (200/200) -> Attack
Is the province of an enemy master and check for castle etc. -> Attack
[Compare Armies] -> run a virtual battle
Do I lose more than 45% (110/200) OR the hero dies -> Retreat
Else -> Attack

Why did I check?

With that behaviour, you have no chance whatsoever to build allies or finish any quest, because the enemy masters will be all over you if you let your heroes look around for brigands and slugs rather than expanding the kingdom by force.

I changed it to only attack provinces if the virtual battle returns NO losses (the green message when you enter a province) and now they kind of expand as quickly as I do.

That means...
On a big map there is usually one enemy master right next to you and for some reason or Murphy's Law, he will go straight after your base and declare war, instead of expanding into the other direction (maybe ~20 turns). After you killed that guy and took over his base, you actual do have some time to do housekeeping and eventually a few quests?

I am talking beginner (default) difficulty. There is no difference in the behaviour though. Maybe the AI does not expand as quickly in expert mode as the neutral forces would be stronger (the province defenders are related to the human player' difficulty)?