"Microsoft's outperformed just about every other asset I could have owned," former Microsoft CEO Steve Ballmer told The Wall Street Journal in an interview published Sunday.Steph Chambers via Getty ImagesSteve Ballmer said his investment strategy is partly influenced by Warren Buffett.But Ballmer, whose net worth is larger than Buffett's, has an unconventional investment portfolio.More than 80% of Ballmer's portfolio is held in Microsoft stock, per The Wall Street Journal.Steve Ballmer has an unorthodox investing approach.The former Microsoft CEO is worth $151 billion, per the Bloomberg Billionaires Index, making him the ninth richest person the world.That puts him ahead of famed investor Warren Buffett by a nearly $10 billion margin.In an interview published Sunday, Ballmer told The Wall Street Journal that his investment strategy is partly influenced by Buffett, who has long said that since most people picking stocks cannot beat the returns of a general index fund.