It all started when Morgan Stanley’s(MS)Apple (AAPL) analyst Katy Hubert and her colleague Internet analyst Brian Nowak pointed out that Apple is spending $3.9 billion on three new data centers in the U.S. and Europe and that this is part of the company’s overall plan to move work away from Amazon (AMZN) Web Services.

That tidbit, based on information from Apple’s earnings call last week, was picked up by Barron’s and other publications on Monday. Morgan Stanley followed up with a brief video in which Hubert and Nowak, who maintained his “overweight” rating on Amazon shares, went through their analysis.

The analysts estimated that Apple is building out 2,544 thousand square feet of data center space, compared to an estimated 6,707 thousand square feet of AWS data centers. That’s a lot of data center.

For more on Apple check out the following Fortune video:

They acknowledged pushback on their contention that 90% of Apple’s IT spending flows to AWS. That is a striking, and some say, unbelievable number. If 90% is true, that could account for about 9% of AWS revenue in 2016. Taking that into account, they parsed the numbers different ways.

What did they find? “Even if only half of Apple’s spend goes to AWS, that’s still nearly $600 million or 5% of AWS revenue,” Nowak said in the video.

To add some context, for its just-reported fourth quarter, Amazon logged a profit of $687 million on revenue of $2.4 billion.

More: AWS customers worry about cloud lock-in

Apple, Nowak noted, may well be Amazon’s biggest customer, but he urged caution. First, there is no indication about how fast Apple may make this transition and even factoring out Apple’s business, AWS grew 80% last year, he said. The two analysts expect the move will take two years.

Those with long memories may remember when Zynga, then a high-flying game maker, transitioned off of AWS to its own data centers. That was a big deal at the time, but AWS forged ahead undiminished.

Apple and Amazon are two companies that aren’t exactly noted for their transparency, but reports over the years have noted that parts of Apple’s iCloud service run on AWS and on Microsoft (MSFT) Azure, which was not mentioned in the Morgan Stanley report. Perhaps that is no longer the case, but if it is, presumably Apple’s move to in-house data centers could be bad news for Microsoft as well.

Fortune reached out to Apple, AWS, and Microsoft for comment and will update this story as needed.

This article was updated at 10:30 a.m. to correct a typo. Morgan Stanley’s estimate that Apple could make up 9% of AWS revenue would come to $600 million, not $600 billion in 2016.