Quoting ben_sphynx, reply 2
There an amount of xp determined by the monster strength and your party strength - call it x.
If you have no heroes, each unit gets x/2, and henchmen get x each.
If you have one hero, that hero gets x, henchmen get x each, and non hero units get x/2
If you have more than one hero, the heroes and henchmen get x/(number of heroes), and non hero units get x/((number of heroes))
They have changed how the first stage is calculated, as compared to fallen enchantress, so that beating monsters stronger than your group does not provide as much of a bonus as before.
Basically in LH a single army is utterly counterproductive and its much more effective using multiple armies with 1 hero with each. In FE single army was the way to go though.
If I'm trying to level heroes, I don't see how this hurts. I still get the same amount of total "hero XP" I would otherwise, it's just split across more heroes. I can also take on stronger monsters than I would normally be able to, netting me even more XP! It looks like based on this formula, having more heroes hurts your other units, but not the heroes themselves. If I send multiple heroes in one army, I'm not netting less total XP (split between them) than if I sent just one of them at the monster, right?