People don't realize that antitrust did not originally apply only to the largest few companies in a market. Sure, it was used to break up Standard Oil and Ma Bell, notorious monopolies that openly used their monopoly powers to charge sky-high prices. But it was also used against companies large and small that were engaging in practices judged "anti-competitive". E.g., "Brown Shoe Co. v. United States (1962) invalidated a merger between two shoe producers and retailers because it would have created a market share of 5 percent that might have tended to “lessen competition substantially in the retail sale of men’s, women’s and children’s shoes in the overwhelming majority” of the relevant markets." 5%! Pretty amazing that a few retailers now control a huge percentage of all retail sales. To be fair, Amazon and Wal-Mart have mostly grown organically, but still.
not so much these days