Half of the US GDP is produced in very small geographic sections of the country. What is up with the rest of the US?

Not a fair question, as I will address below...

This graphic has been passed around the Twitter-sphere so much I do not know its origin.

Quite amazing if accurate. Roughly 20 metropolitan areas in the US are responsible for 50%, or about $8.4 Trillion of the US GDP in 2013 ($16.8 Trillion nominal dollars).  

One the one hand it makes sense because these areas have very high population densities and much of the US is relatively rural.  

Still, kinda looks unbalanced to look at it like this.