I get this more on horses than on the bike, because 99% of the time I am on my own on my bike, but the shout is usually the same "you should be riding single file". I can never actually figure this one out, as if you are riding (horse or bike) two abreast, then you are effectively a shorter, but wider road user. However, if you ride single file, then you are half as wide, but twice as long, and as Rule 162 states that a driver must give us the same amount of room as they would a car (and to be fair, most do), then in single file we are more difficult to overtake, as the driver need more distance.