Possibly all
Many consider the 13" models to be the near perfect size. This even as Apple has always shorted them in comparison to their larger 15" siblings. For instance in having to settle for duel core versus quad, and no dedicated graphics.
Others want more screen real estate, so ignoring the 13" models to begin with. And just a happy coincidence or not that in paying the higher price they also end up with a significantly more powerful notebook. Some of them are also masochist enough to claim that lugging the 15" models around is no big deal, and anyone complaining in thinking of a smaller size should hit the gym. Nevertheless, some of these Schwarzenegger's in training do wear out in time, and decide that they do not need all the horsepower of the 15" and would prefer the smaller size and weight of a 13". Each will have their own reason, if it often comes down to just that.
As for right now, it is hard in cost and performance terms to justify the 13" retina versus the 15". Max out the 13" and one is verging into 15" price territory, but without its capabilities. With the advent of Haswell this autumn that may more even out. But for now, even with Apple's recent price reductions, you'll be paying a premium for a retina, and especially for the 13".
But it might be all you want.