I have always ridden 700x23C tires, which don't weigh that much more (empty) than 700x20C tires. But then I noticed how much of a wheel's weight comes from the air at 120 psi. Since they're thinner, inflated 20C tires should weigh quite a bit less than 23C tires. This difference is in rotating weight, which counts! Why don't more people use thin tires? Are they really uncomfortable? More prone to flats?