Well in the earlier days most hot dogs and sausages used natural casings either sheep for smaller diameter or hog for larger.  Today many are made using a plastic casing, then that is stripped off the hot dog after cooking, so the diameters now are up to manufacturers preference, a slightly smaller diameter gives the illusion the hot dog is the same.  Unless your buying by weight, they can reduce the diameter slightly and give the illusion your still getting the same amount if sold by quantity.