Power consumption is probably not a good proximate for disk capacity. Dense disk capacity has way lower power per rack than compute. Unless you know the overall compute/storage ratio, very hard to figure out storage capacity. The reverse might work if compute power consumption overwhelms storage power consumption. On the other hand, it might be easier to infer storage than comoute from rack positions, or square feet of dc space. Only so many spinning disks per cubic meter.

Also, there's a Reinvent talk from several years ago with "8 exabytes" in the title, so surely Google and Amazon both have many exabytes by this point.