This was many years ago, for horse-riding. But we were always advised to have 2 side by side- around the width of a car. Which seems the same principle for cyclists...
To make it a shorter space for overtaking, and to prevent motorists trying to squeeze past.
The worst thing in a car, is when cyclists are single file, but spread out, so you have to overtake just 1 or 2 each time.
Common sense would say to keep as compact as possible, but on narrow country roads, with just one car behind you, move in.