Well, generally speaking, traffic jams are caused by "stupid" people, it has nothing to do with what kind of vehicle they have, just how they are driving it.
The size of a traffic jam however is somewhat related though, to the size of the vehicles in that traffic jam, I'm sure you'd agree that a traffic jam of say 250 schoolbusses would takes up a much larger chunk of highway than a traffic jam of 250 Geo Metros.
As for the idea that high fuel economy leads to increased congestion - take a look at other countries that have a higher ammount of smaller cars, and motorcycles etc. as compared to the U.S. which has few motorcycles, and more larger cars. Most of them don't drive any more than we do here.