I've heard this attributed to the reworked EXP All. In Gen1, EXP All evenly distributed all XP points received. It's basically like switching to every Pokemon in your party, except you don't have to waste turns doing it. In Gen6, EXP All copies the normal XP from defeating a foe to all of your other mons, effectively sextupling the amount of experience points you receive. Turning it off greatly increases the challenge, I've been told. I didn't find out until I was almost finished with the game that it was doing this.
I've only played Y once, so I could be completely wrong about the increased challenge. I've been thinking about getting a copy of X, or maybe Z if it ever comes out, until I remember how badly the Fairy type screwed over the type charts. I really hate digi mega evolutions too, but at least those can be ignored in the solo game. X/Y had this weird thing where it seemed to be trying to appeal to older fans of the series from the Gen1/2 days, but then it does things that seem to exist just to mess them up, like retrofitting well-known Pokemon with new types that resist their old counters and changing an item in such a way that it eliminates all challenge the game has.