You should be aware that there is a $300 difference between the two machines, right?
I think he is talking about the return on your investment in terms of performance. The six hundred dollars separating the base 13 and 15 is pretty hefty, but if you are going to spend 300 more, why not go all in? I think that is the thinking.
The base 13" is a really good deal (I think most people would agree). The more you add to it, though, the more affordable the 15" is going to start looking.
For the higher end 13" you will pay 300 dollars more to get a little more processing power and slightly larger hard drive. But, now you are getting closer to the 15", which will give you considerably more processing power with quad cores, the larger hard drive, along with better screen resolution and more screen real estate.