Originally Posted by
JohnDThompson
Off-road frames often have curved chainstays to accommodate wider tires, and need the crank arms positioned further out to clear the stays.
Yabut…
It depends on the crank design. While mountain bike frames are wider, some mountain bike cranks use narrower spindles than road bike because the arms curve inward towards the frame more than road cranks do. For the Sugino cranks in question, the recommendations are correct but they might not be for other cranks.