Once again this depends on the AI and what a stack means.

For instance in MTW1 it was not uncommon to have 5-6 full stacks attack a small nation. In MTW2 your average is one half stack against another half stack. (excluding mongols and crusades).

I mean for example, if everyone is out colonizing, and ireland pops up with 2-3 full stacks, they should not be able to march all the way down and take england.

The build up should be enough to conquer a province/city or two, and scare the home country a bit.

Of course true conquest, shall occur

IN THE DEMO