1. Numbers don't matter that much because most of the time you don't have to defeat their stack with your stack. If you can't beat their stack, why, don't fight it! Assault/siege instead and mislead their stack.
2. If 3000 men could beat 6000 men due to factors mostly out of your control (whether there's tactical genius you can use, supplies, morale), then CK2 would SUCK. Maybe it would be fun for you for the first 20 hours of gameplay, but when you get larger, hell, you don't want to micromanage each army. That is totally out of the question.
3. If YOU can come up with some combination of influences on how to beat 6k with 3k men, then the AI should be able to do it consistently. And that would be just awful. Because either the AI can't because it's too complex to program, or because suddenly a silly revokation war means a 20 hour planning fest.