In looking at the Wikipedia article, I think it shows 2.3" difference between a typical 26" and 29" tire.

You would think that if going from a 26" to a 29" tire made a big difference, then we'd all be riding Coker Monster Cycles, and all the top mountain-bike riders would be using Coker 36" tires on their bikes.

For years, the European standard was 28" tires and the American standard was 26" tires. Maybe the Europeans were stupid for using those big tires. Maybe we were stupid for not using them. Or maybe there's just not really that much difference between the two.

By comparison, unicycles don't have available a cheap gearing method, so increasing the tire size is one way of going faster. Result: Coker 36" tires are very popular there, with several different companies making the unicycles to fit them.