Wal-Mart Stores Inc. (NYSE:WMT) plans to speed up mid-sized Walmart Market stores openings, said U.S. stores chief Bill Simon. The company plans on opening 90 to 100 of the mid-sized stores by the end of the year. In addition, it plans on having 300 by fiscal 2013 Simon said at the William Blair & Co. Growth Stock Stock Conference.These mid-sized Wal-Marts will be roughly the size of a supermarket, around 42,000 square feet. Simon says Wal-Mart is expanding the format because its return on investment in the stores is that of the larger supercenters Wal-Mart is known for.Wal-Mart has also introduced smaller, quicker shops for the customers who want to finish their shopping quickly, called Walmart Express.Wal-Mart Stores has a potential upside of 18.8% based on a current price of $52.12 and an average consensus analyst price target of $61.89.