This is related to extreme perk bonuses. There are 4 perks giving xp.
1-Combat Tips : +15 xp per troop / day
2-Raise The Meek : +30 xp per troop / day for troops < tier 4
3-Walk It Off : +15 xp per troop / / day while traveling on map
4-A Good Day's Rest : +30 xp per troop / day while waiting at settlement
These xp bonuses are given per troop. It was per stack previously and changed to per troop. Of course per troop makes more sense but values did not reduced while changing to per troop. We are fixing it (reducing daily per troop effects to 3-5 instead of 15-30)
The amount of XP for Raise the Meek seems perfect right now. It lets you get low tier troops up to speed at a pretty nice rate that doesn't feel too fast, but its not too slow either. Nerfing it to 3-5xp will just make the perk useless again. The only problem is that its supposed to only work for Tier 1-3, but now it seems to be giving xp to all tiers.
I've also noticed that the AI seems to get much more xp from battles. When fighting alongside my companion's parties, they always get around 3-4x more troop upgrades after each battle than I do.