Here's what happens and why you don't see any huge leaps in stats: it's because you only get

*all* the stat points from EV training when your Pokemon is level 100. Otherwise, the amount of stat points you get for a particular stat depends on the level. Smogon's EV training guide is the best and the one I used to understand the mechanics,

here it is.

Copied from there:

So basically, the amount of stat points you get is actually (EV/4) * (Level/100).

Also, if you want to check if you've done the EV training correctly, call Bianca in B2W2 and she'll tell you if your Pokemon got 510 EVs in total or 252 EVs in a particular stat.