It refers to the fact that many operas do not end until the star, commonly a soprano, sings the aria that is the climax of the show. In the past, many sopranos were overweight because larger size was thought to give a person a better voice.. . The phrase was around before Monday Night Football, but it was popularized by its use on that program. Maybe it was Fran Tarkington who used it there, but I'm not sure.