Originally Posted by
Aysha
It is indeed capped by leveling.
If you turn in a 10,000xp Assignment in on a Level 1 character, you will get the 500xp, ding Level 2 and then stop 1XP shy of Level 3 and end up wasting almost all of the XP. So, those awesome Legend Rank Assignments aren't even any good to do until you're Level ... 40? or so. And even then, you need to throw something else in, in-between the assignments to ding now and then so you aren't wasting half of the XP.
You would gain levels far faster by doing the Grounds Tome in the Crawlers' Nest until you're about 50-60 then you move onto Lv100+ mobs from there. That is, of course, unless there's a faster way I've yet to discover, like maybe Abyssea or something, never tried that. But IIRC, Abyssea requires Lv30 anyways.