Currently each wargoal have a set amount of piety and prestige gained and lost which is when divided between the Victors. I don't think that make much sense, for example if a weak realm defeat a much more powerful one it should gain alot of prestige and piety while a large realm stomp an small one it should not seen as particular prestigious or pious.
Thus one way is to base the gained/lost piety and prestige from a Peace deal on the relative strength between Alliances so if the weaker side wins they gain alot while if the stronger side wins they don't get much.
Thus one way is to base the gained/lost piety and prestige from a Peace deal on the relative strength between Alliances so if the weaker side wins they gain alot while if the stronger side wins they don't get much.