I can't explain the science behind it, but its been my experience that a smaller wheel allows faster acceleration, but it's less efficient once normal riding speed is reached if you only consider the time it takes to go a certain distance. My thinking is that the taller wheel allows the bike to travel farther for each rotation of the wheel.

Also, the smaller wheel seems to take less strength to get up to speed, but requires more effort to maintain the same speed as a bike with taller tires.

I saw a similar effect when I replaced my car tires with lower profile 70s. it took a higher rpm to maintain the same speed in any gear than what the higher profile tires required. The change in tire size affected the gearing ratios.