BENTONVILLE, Ark. — Walmart plans a modest acceleration of its small-format stores in the coming years as investors were assured Wednesday morning that financial returns are now comparable with the company's large supercenters. Walmart kicked off its 19th annual investor conference on Wednesday with optimistic comments by senior U.S. leadership about their outlook for the holiday season as well as an outline of growth initiatives for 2013 and beyond. One of the most highly anticipated revelations heading into the event was Walmart's plans for its smaller format Neighborhood Market and Walmart Express stores. By year end, Walmart expects to be operating a total of 240 Neighborhood Markets and 12 Express stores. By 2016 the number of Neighborhood Markets will double to roughly 500 units generating $10 billion in revenue, according to Walmart U.S. president and CEO Bill Simon.