Actually, the wider rim is supposed to create more of a U-shaped tire that has a better contact patch and lower rolling resistance versus the narrow rim making more of a lightbulb shaped tire with a worse contact patch and higher rolling resistance.

This from the Hed cycling website addressing the advantage of wider rims:
In 2006 Steve became rather dissatisfied with the interaction between the rim and the tire. By this time the 23mm tire had become the standard choice for nearly every cyclist. However, Steve noticed that combining a wider tire with a narrow rim simply did not make any sense. Aerodynamically, the narrow 19mm rim and a 23mm tire created a light bulb shaped profile. Needless to say, this is just not very efficient and leads to increased wind drag. Steve simply invented a solution and Hed's C2 technology was born. C2 technology has its maximum effect on clincher tires by solving the problem of the tires requiring extreme pressure to keep them from flopping over the rim during a turn. This high pressure also leads to deficiencies in cornering performance and comfort. By making the rim edge wider, C2 allows the tire to flow almost perfectly into the rim shape, ushering in a revolutionary step forward in aerodynamic performance. Although, the benefits of C2 design go even further. The wider tire mount also creates a larger contact patch with the road which distributes load better and lowers speed-sapping rolling resistance. All said, C2 technology improves aerodynamics, increases the road/tire contact patch for better cornering grip, decreases rolling resistance, and allows the wheel to be more comfortably ridden at a lower tire pressure. Not a bad improvement.

Also, more info here: http://flocycling.blogspot.com/2011/...art-4-flo.html


On the left is the wider rim, the contact patch is wider but shorter, whereas on the right is a narrow rim with a narrower but longer contact patch. The longer contact patch can increase rolling resistance.