A crashing chandelier, a turntable barricade, a life-size helicopter—if these dazzling set pieces evoke misty, water-colored memories of “The Phantom of the Opera,” “Les Misérables,” and “Miss Saigon,” then you already know all about megamusicals. As the name implies, the early megamusicals leaned into a “bigger is better” mentality—from the budget to the cast size to the elaborate staging.