Costco Wholesale Corporation (NASDAQ: COST) and Wal-Mart Stores, Inc. (NYSE: WMT) have come to dominate U.S. retail. While that size is a testament to the company’s historical success, lately it hasn’t been much of advantage. Shopping habits are shifting as more people buy online, and Amazon.com has been rapidly outgrowing Costco and Wal-Mart lately, and even surpassed Costco in revenue last year.