Guys, it really depends on what exact kind of an RPG a game is.
DOS is clearly one of those that are heavily focused on action and combat as its main feature.
Pillars of Eternity clearly isnt, or doesnt want to be so focused on actually killing enemies as the main source (ahem) of XP.
New Torment will do that even less. For example.
Back to OS, considering its focus on action and combat there are several quest solutions that give very big rewards for different diplomatic solutions. But this game cannot be a different type of an RPG then it is.
AND most importantly, you cannot expect that every solution and every choice will give you same amounts of XP or other rewards because that completely removes the core feature of choices and consequences - and that makes the game less of an RPG.
Because then there is no difference to anything you do (including different choices in skills and stats) and that is an action game. Not an RPG.