
One might argue that Japan's Yomiuri Shimbun, with 14 million daily circulation, is the world's largest newspaper. Metro says it has 18 million readers, but readership is different from circulation. Circulation is the actual number of papers bought or distributed, while readership reflects the total number of readers. Frequently, newspapers will claim that 2.5 people read each paper, which inflates the paper's "number." Metro's total daily circulation has not been disclosed.
It's possible that the folks at Guiness ignored Yomiuri Shimbun because the title Metro received is "World's Largest Global Newspaper," and Metro, unlike the Shimbun, is truly global with 70 editions in 100 cities, in 21 countries and 19 languages in Europe, North America and Asia.