You don't have to run fast to be a real runner! John Bingham
The Clydesdale category emerged in the 1980s after a Baltimore accountant analyzed runners in 10-kilometer and marathon races. He found that after men reached 170 pounds, their performance declined relative to their weight. This meant that a large, slower runner might actually be performing better than a smaller, faster runner. As a result, some smaller races began offering the Clydesdale category. Eventually the category was adopted by larger races such as the Marine Corps Marathon and the Portland Marathon.
The Clydesdale weight category serves a purpose similar to age and gender categories -- to allow competitors to compete with others who are at a similar running level. Jeffrey West, a 245-pound marathoner, told the "New York Times," "As a Clydesdale, you know you’re never going to win a race." But these larger competitors can compete against each other. This gives larger runners a competitive incentive in the race.