Jan Hein of the Bicycle Quarterly is the guy who has changed everyone's perception of tire sizes and pressures needed. It is interesting to read his scientific experiments that led to the bigger tires and less pressure. His evidence is so strong that the Tour de France riders are now riding bigger and bigger tires with less and less pressure. Jan's evidence shows that it is sidewall flex that determines how efficient a tire is to pedal and bigger is better in all cases. Pressure has a big role in this. He has experimented up to 50mm tires that role easier than 25mm tires at over 100psi. I have changed my tire pressure habits and now experiment with pressure to find the sweet spot for the tires I'm using. My smallest tire on a road bike is 32mm at this time and I'm very happy with them.