For armies there exists an additional experience gain for leadership.
The formula is: Army Strength x Morale x 0.0004
Morale is the average morale of the army and army strength is calculated based on the unit composition(tier weighted unit counts, e.g. T2 unit is ~0.9) of the parties making up the army.
Bonus Answer:
We will balance the leadership gain since after removing a bug making leadership giving enormous amounts of morale, the average morale is rather hard to keep at high enough levels to sustain experience gain.
On the topic of "armies without kingdoms", it's definitely something we're considering (to include or not to include). I am not sure how to implement without breaking early game balance as armies without kingdoms can lead to a kingdomless player to overpower some weaker factions or rebel clans and create a pseudo-kingdoms without all of the necessary responsibilities of a kingdom (like voting, requiring influence to make people do certain things, etc.) but with large benefits such as taxation direct control of the fief ownership and similar.