MicroStrategy has sold a larger-than-expected $2.6 billion of bonds that convert into stock, giving the cryptocurrency-buying software company more cash to purchase bitcoins. The convertible bond offering was increased in size by almost 50%, from a previous $1.75 billion, the company said Wednesday, signaling strong investor demand. The company unveiled an audacious plan last month to raise $42 billion of capital—half in stock and half through bonds—over three years to buy bitcoin.