My Jen was measured at a flyball tournament once and after they finally got her to stop wiggling
, she measured at 15 1/2 inches. That made her a height dog, easy. There was a crowd waiting around to see, even though we were almost the last in line that morning. You can just imagine the conversations I had at that tourney from people who didn't actually know me (certain people knew better, lol!).
Her littermates ranged from her, to smallish/normal to normal, to one 22 inch dog at least (I think he wasn't the only big one, either).
That's the thing with the little Border Collies, they are often one-offs, so it would take a lot of culling to get the size down consistently using just purebreds. Narrow that down to just dogs that work even to a standard of basic competence, and are healthy and of sound temperament, and you'd have a nearly impossible task.
It's a good example of how even one physical standard makes it difficult to select for sound working ability.