When are hooves "too short"?

Someone on the UKNHCP forum was asking whether their (newly out of shoes) horse was sore because he was "wearing his feet too short". Its a common question, and there is a common preconception that barefoot horses will "wear their feet away" doing road work. None of us with barefoot horses have ever seen this happen, but its still a pervasive myth :-)

But back to the horse who was sore...It turned out that he had been working well without shoes and had been fine doing roadwork right up to the point when he was trimmed. In fact, it was the trimming of his sole and frog which had made him sore, not working without shoes. And before anyone asks, it was a registered farrier not a UKNHCP practitioner who performed the trim.

These photos aren't of that horse, but they do show a foot that is way too short - not because its got little hoof wall, as people sometimes think, but because its internally too short - there is too little depth of sole protecting the internal structures of the hoof and its a sure way of making a horse sore.