It is just that the 13" is extremely capable and should handle pretty much anything, the 15" will just get it done faster. There really is almost no task that is impossible to do on the 13" these days, and that is partly due to Intel really producing amazing CPUs.
I must admit, from what I have now seen, the dual cores used in the 13" MBPr's are really powerful. I suppose it is also the all SSD route that is making them feel fast too.