Apple’s large-screen iPhones are a big hit in China, taking market share from Samsung and selling at a pace that may make China a greater source of revenue than the Americas for Apple in coming years, analysts said. The world’s most valuable consumer electronics company reported on Monday a 71 percent increase in sales in China to $16.8 billion, driven by its new, bigger iPhone 6 and 6 Plus. In the first three months of the year, for the first time, Apple sold more iPhones in China than in the United States.