There are no differences in rolling resistance between 650 and 700 tyres related to their respective sizes. Although obviously a 700c/23 Conti GP 4000 is going to roll faster than a knobbly 26" mountain bike tyre!
I'd say there
are differences, but that they are mostly irrelevant -- 26" wheels commute and "roll" just fine. Most cyclists would probably be pressed to tell the difference among common tire sizes.
A mountain bike with ~ 1.5" tires will make a great commuter. Most people who swap to 700c (if possible) and notice a performance increase are probably just noticing that their 700c road wheels are lighter and are comparing 700c skinny tires to
knobby mountain tires. Smooth mountain tires (26") roll fantastic to me....no need for 700c.