That seemed high to me. Looking at the financial statement for Q1 2017, AWS is 3,661 million USD in net sales out of 35,714 million in net sales overall. I don't know if Q1 is unusual but this is around 10% for AWS.
I came here to say the same thing. AWS is definitely not a quarter of their revenue.
I think AWS is the majority of their profit but not revenue, it basically allows them to subsidize the rest of their business to gain market share off the absurd AWS profit margin.