First of all Quest EXP is proportionate to the difficulty of the quest. Goblin King Kong has a high amount of EXP but is not the highest, for that Boss Party is the highest because it is the hardest. You can buff the EXP you receive in other quests by changing the Difficulty of the Quest and/or sacrificing an item to make it more difficult and thus increasing the EXP gain. Other than that, grinding is a part of questing.