O/U's have longer barrels mostly to increase the length of the sighting plane. The longer barrels add very little to the muzzle speed of the shot, virtually all the shot speed is developed in the first first 20 something inches of barrel.
Autoloaders have shorter barrels because the receiver makes up part of the sighting plane. A 26" 1100 is almost as long as a 30" O/U.
Remington once made some 34" 1100 barrels. The overall length is about the same as an O/U would be with 38" barrels. Not very handy but these barrels bring a premium price to those that love them.