Walmart CEO Doug McMillion agrees that the federal minimum wage of $7.25 an hour is too low. Going all the way up to $15, however, could disadvantage small businesses that would be hard pressed to keep employees on and compete with that labor cost. Mr. McMillion, speaking at a Business Roundtable event, said he favored a higher minimum wage plan that took “geographic differences” into account. Walmart pays different starting wages to workers depending on where they live.