After decades of expansion, Wal-Mart Stores Inc. appears to be bumping up against the limits of the supercenter model that's made it the world's largest retailer.<br />Pressure from Amazon.com Inc. and a growing cast of smaller retailers, from regional grocery-store chains to dollar stores, have put pressure on the big-box model Sam Walton practically invented back in the early 1960s.<br />The challenges were driven home Wednesday when a historic drop in projected profit for next year sent Wal-Mart shares into their biggest one-day free fall since 1988.<br />Wal-Mart Chief Executive Officer Doug McMillon is sending a clear signal to Wall Street: near-term profits will take a hit to make the massive investments needed to reinvent the retailing giant at a critical juncture in its history.<br />The company plans to invest heavily in its online offerings as it opens fewer of its massive supercenters, focusing on smaller format locations called Neighborhood Markets.