Walmart has both the largest number of stores — about 90 percent of Americans live within 10 miles of a Walmart — and is the leader in market share, followed by Kroger, Costco, Albertsons and Amazon. The supermarket industry could further tighten if a proposed $24.6 billion merger between Kroger and Albertsons is allowed to go through. A federal judge in Oregon is currently deciding if the merger will help or harm consumers.