Dwarfism for corgis is also a functional trait - being so low to the ground meant that cattle would have difficulties kicking them in the face (which they evolved to do against predators that notably did not have short legs, putting their hooves at perfect head-kicking height).

Of course, when they aren't herding animals and therefore not are risking a hoof to the skull, dwarfism comes with its own set of problems that they otherwise wouldn't have.