Mika you gain levels easier in X & Y because you get that boost from Pokemon Amie if you use it. So they level up higher plus the fact that the EXP share is for ALL pokemon in Party and not just one. Then the exp isn't split anymore so anyone who goes into the fight gets the Full exp of whatever you are battling. Though that is a pain when your just trying to evolve something for a Dex.
In other games there is alot of Grinding evolved and the level limit before first gym is 20 which when your training weak pokemon it's hard to not go over it.