Is there a penalty to fighting weaker armies?

[IntelCoreDuo]

So I gather that dragon level is dependent on leadership, but will gain more leadership if lagging behind hero and will thus remain at about the same level. That makes sense, otherwise people who only use dragon spells that do no damage and have long sleep time would end up with very low level dragons in late game.

But how about the hero? Should I reduce the number of stacks or units to gain more XP and level quicker, or do I get a fixed amount of XP from each battle, ignoring bonuses etc?