That's really interesting.... and now you mention it, our rare breed Balwens don't ever herd in a tight bunch.... I'll often get a general 'gathering together' if moving field, but often little sub groups... I saw some neighbouring mules in an adjoining field the other day, over a hundred of them, and they were in this remarkable (to my eyes!) tight huddle!
In part I'm sure, as you say, it's to do with modest flock size (we're 30-40), but also maybe with a small 'hobbyist' flock, on a smallish plot, they just feel very safe, and so less need to flock together?