Rolf at Hostel Shoppe said that larger diameter wheels allow recumbent riders to keep up, when riding with a group of fast road bikes. He said that the smaller wheel diameters (below 26"/559mm) are slower, and that riders were slowed down enough by them to make a noticeable difference.
But I have also heard conflicting reports. The world-record-holding bikes for speed (when drafting behind a lead vehicle) use smaller diameter wheels. Also, Alex Moulton has talked about his decision to use smaller-diameter wheels, and about their greater efficiencies, and about the fact that trains, airplanes, and cars all went from larger to smaller wheels as they evolved.
So, what I'm wondering is if anyone here has anything to add that will help clarify this.
(Between 700c, 650B, 650C, and 26" (559mm), there seems to be very little difference. When you drop down to 20", though (406mm), some people claim a significant difference....)