A lot of that was certainly just for the root volumes of all those ec2 instances (how much exactly is hard to know without more details). Which of course would have duplicate copies of the various base images for the VMs.
Although, that does bring up the question of why AWS doesn't have a way to share a single read-only volume across multiple ec2 instances in the same availability zone. In many workloads there isn't any need to write to disk.
There kind of is, but it's not really made for that use case so there's a bunch of caveats (it's read/write, has a limited max number of attachments, io1/2 required, can't be the boot volume): https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ebs-volu...
Although, that does bring up the question of why AWS doesn't have a way to share a single read-only volume across multiple ec2 instances in the same availability zone. In many workloads there isn't any need to write to disk.