Yeah but why should NPC lords waste their time and money chasing a few peasants with big parties? That's pretty stupid thing to do. What if them NPC lords behaved a bit more like... real world?
TW obviously decided not to code it so that AI Lords behaved realistically. If it was a design decision or more likely simply not worth the effort for something most people playing would not notice until they have completed some campaigns...
Making smaller parties have to survive a skill check by larger parties would be worse than the current situation.
As it stands, you can run a small party and outrun or dodge into towns against enemies. If you run a mid-size party 30-80 there are enough perks you will generally be able to catch most of the smaller AI parties but caravans and some parties will simply outpace you.
If you run large parties or armies, you will barely be able to catch anything at all. It is already an problem where you have an army of 1000 when an enemy army of 2000 approaches that has 2-3 smaller armies around it that initiate the battle and the larger army catches up. Not sure if the Ai does this via code or just happenstance when the larger army gets into reinforcement range (if you abandoned a siege or completed a battle with less room to run than guessed or something.
Having an ambush ability which require high tactics, leadership, and roguery might be handy so larger parties could catch smaller parties but making such a mechanic universal would cause much more rage than how it works currently.