The areas are generally about the same difficulty, relative to the level that you have if you've done all of the main and side quests, are getting good equipment, and are using the spells that you get.
If Mooshu is easier, then it's possible that your equipment, spells, etc. are better than they were, relative to what you're fighting.
Mooshu won't stay that easy anyway. There are various bosses who will give you a bit of a challenge.