The problem is you would end up encouraging players to build with the same blocks because they give you more xp. Actions should only be earning a very, very small amount of xp and not as a way to level up quickly. The feats will be giving you big payloads for preforming actions and achieving a milestone in the game.
Objectives are intended to give you big payloads early on then fizzle out when you have learnt most of the features in the game.
Contracts is where you’ll get bigger payloads, these will be randomly generated quests at varying difficulties and tasks to complete. If you are looking for a way to maximise your XP gaining, then a combination of actions, feats, objectives and contacts is the way to go.