I've designed industrial mechanisms, but not bikes. Sometimes, over-spec'ing the motor makes sense. You don't have to feed it all 2400 W. But the bigger motor might be easier to cool, have other advantages such as more convenient mounting and the right size spindle, and so forth. Counterintuitively, below a certain size, motors start to get more expensive. And using a catalog motor avoids having to spec a custom motor in order to get everything you actually want.