It's simple math actually (and I'm terrible at math so this is basically common sense stuff). Whoever starts out the strongest is going to have the most troops and best equipment at any given point and time months down the road.
If you're building up while everyone else is going to war, you're going to have more troops and more gold.
This is all common sense. The problem isn't DRZ or alliances, the problem is a game mechanic which doesn't reward people for fighting for fiefs. It doesn't punish people for having massive amounts of people in the faction, or controlling many fiefs. The more fiefs you control, the more troops and gold you can make (potentially).
Strategus mechanics are the problem, not people who actually use strategy and diplomacy to benefit themselves.
There's been dozens of great suggestions for "fixing" strategus over the last 3 or 4 months. I'm not going to even begin to list them all.