Some sidequests are very quick, regardless of only being able to be completed once, I don't think they all justify being increased.
But they are all a uniform exp reward, they could probably do with being split into tiers. Quests that take you to one location and back in 40s just give you the 150k exp, then the multiple location ones give you 200k, and the more important sidequests with proper cutscenes give 300k.