Exactly, the storage capacity must be lower. If the data center uses 65 MW and a single hard disk with all overheads 5W, there can be at most 13 million of hard disks there resulting in the maximum capacity of the order of exabyte, not zettabyte, or for the given example, at least 100 times less.
That gives "only" an order of (17e4/0.019) ~ 10 million times bigger area for the file cabinets than the Stasi files, or the square 3000 times bigger in every direction compared to the Stasi one.
While I agree that they likely store much less data than five zettabytes, your assumption that all their storage is on constantly spinning hard disks seems flawed.
(Tape libraries and various other offline/nearline storage solutions would be viable for most of their data)
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