Wal-Mart’s appearance for the third time in a row at the top of the Fortune 500 list of largest U.S. companies would seem to confirm what many have long suspected: This retailing behemoth is all-powerful and unstoppable. The company, based in Bentonville, Ark., has reached such enormous size that it has been compared to General Motors in the 1950s and Standard Oil in the early decades of the 20 th century. As such, its influence in the U.S. economy is being reevaluated by politicians, economists and the media. Wal-Mart’s overpowering presence in the retail industry has polarized public opinion about its impact. For example, W. Michael Cox, chief economist of the Federal Reserve Bank of Dallas, told the New York Times recently that “Wal-Mart is the greatest thing that ever happened to low-income Americans.…