First of all, I love the idea of somehow buffing up the mobs of FATEs based on the number of players participating, probably hard to do but it would alleviate the problem of being able to zerg rush through FATEs in no time at all.

That being said, FATEs of X level should give roughly the same experience as quests of X level, but right now they give significantly more. I don't really think this will ever be changed (decreasing the speed of leveling will never ever happen after an MMO launches) but it does seems like an odd design decision.