As boxer Andy Ruiz Jr. prepared to step on the scale during the weigh-in Friday afternoon for his bout against U.K. heavyweight champion Anthony Joshua, he heard a taunting chant from Joshua’s rowdy British fans: “Who are ya? You fat bastard!” The flabby Ruiz, who is listed as 6-foot-2, smiled and flexed his biceps as he tipped the scales at 268 pounds. Ruiz’s dream, of course, was to win the heavyweight title, which he pulled off in shocking fashion, stopping Joshua via a seventh-round technical knockout on Saturday night at Madison Square Garden that instantly stands as one of boxing’s...