You can't compare a 700cx23 to a 26"x2.1, they have completely different uses. If you look at what experienced riders tend to use for urban commuting, in 700c that is 28-32mm and in 26" it is 1.25-1.75 width.
There is very little difference in practice between a 700x28 and a 26" x 1.5. Both are fast enough and rugged enough for the job and take roughly the same tyre pressure.
The main issue in wheel efficiency is tyre design (slick/semi/knobbly) and pressure. Diameter is more theoretical than practical.