Fatbike owners seem to be fond of using lavishly wide handlebars. They also don't just go from 640mm or 660mm to 680mm or 700mm, but often to 720mm or 740mm. I'm assuming their shoulders couldn't be that wide, and they choose this width for control, even at the risk of having a posture typical of motorcyclists rather than cyclists.
Maybe this is due to the force needed. After riding a fatbike a bit, I'm surprised at the amount of force it takes to return to straight. During a left turn, for example, the handlebar exerts quite a strong pressure. One needs to push with the left arm, or else the bike would turn even further to the left. Likewise, returning to a straight line takes significant pressure.
This is very different from a road bike, where only a hint of an effort needs to be exerted with the arms to turn and to return to a straight line. The hub on a fatbike is only very slightly in front of the line of the steerer, whereas on a road bike or MTB, the hub is significantly in front of that line. On ancient bikes a curvature at the bottom of the fork illustrates this very well. Perhaps the position of the hub on fatbikes almost right under the steerer is both necessary and requires such pressure during turns.
How do I determine a suitable size for a fatbike handlebar? Is it established that it needs to be unusually large to make steering easier? Once the handlebar exceeds 720mm, it becomes a nuisance to pass through doors—a convenience in the winter when all work needs to be done indoors, and so I'm wondering whether such a width is compelling.