Bigger tires also mean dragging around a lot more weight. For short durations, that’s probably not a big deal as was shown in the tests. But for longer rides with lots of climbs, the extra rotational weight can become a factor. That’s why World Cup XC racers are still running <2.3 tires and that is where rolling resistance matters the most. Some of the Specialized factory racers are running 2.1 tires and that is a team that does a lot of testing. Also, for off-road riding, tire pressure, tread pattern, and tire suppleness also matter a lot.