The official Breed Standard
Breeders of Purebred Dogs choose the dogs they want to breed based on manyh criteria, including health, attitude, and physical form, this form is defined within the 'Breed Standard'. Australian breeders use this standard, it is based on the Country of Origin of the Clumber, the United Kingdom's written description and is markedly different from those used in Canada and USA.
on that page you will also find a link to the Breed Standard Extension which explains why and how this type of physical structure is of great benefit for the breed to perform the work in the environment where they were developed.