Smaller wheels have more rolling resistance, that's why racing recumbents have dual 26 inch setup.
Check out the performance bikes at Bacchetta.
The idea does not make sense but there is a rolling resistance formula that I found in a book called "Bicycling Science" by Whitt and Wilson from MIT press. Turns out the smaller wheel have less wind resistance and are lighter, but this will not make up for their greater rolling resistance.
I prefer a 20 inch front wheel becouse it allows for a lower bottom bracket posture.