@ Tharn: Sounds like you are being challenged. That is good, this is a strategy game after all, it feels good doesn't it? Sometimes, if you are trusted in the world or liked by a faction, you can get away with somethings. Be warned however, once you cross a faction or even worse an ally and get an untrustworthy reputation. You better "batten down" the hatches. For life will get very difficult. Yes, smaller stacks come with a more aggressive AI. The higher the level, the smaller the stacks.
Which brings me to a huge design point of this game. Huge stacks are over rated!! here are the reasons why. One huge stack cannot effectively cover the map. As you guys have noticed with XAI and especially KGCM. There are NO stuck units on this glorous map!! NONE, zero, zilch. Also the AI is constantly patrolling borders, building towers etc. The map is very active. We have all seen the AI intercept stacks when at war, run counter offensives. On a Big map this is enormously important. Especially in large areas like Russia. Smaller, and more stacks are much more effective than fewer larger stacks. This is because of the Zones of control they create.