Though I cannot recall where it was posted specifically, the developers confirmed that the AI will lower unit maintenance to help balance budgets and consequently it drills its armies less. You may notice that the Ottomans, for instance, have less army professionalism than usual.
This sentence marks the end of what I KNOW and gets into speculation.
If I had to guess, I would assume that the AI only drills now with large surpluses when it can afford to do so and wants to buff its army, since it mentions in the dev diaries and changelog that the AI considers drill in combat and army quality now, which it in turn uses to interpret its probability of success in wars. I assume the AI raises unit maintenance but does not drill if it feels it has a decent chance of being attacked, like the HRE minors that are constantly surrounded by rivals and allies of rivals. However, the AI still will not drill in this instance since it lower morale and raises unit maintenance, which makes its armies both expensive and vulnerable. Thus I would only anticipate large countries drilling in peace time with stable economies (for the most part).