According to Gartner (https://www.gartner.com/newsroom/id/3884500), the 2017 revenues were:
Amazon: 12.2B
Microsoft: 3.1B
Assuming Amazon continues on the same 25% YoY trend, it would be 15B in 2018. Microsoft could beat that number by reaching a 400% growth rate. That would be pretty amazing. But it's not the case.