Snapshots of Rcs.iss are a best estimate based on the disk usage. Snapshots of volumes operate at the block storage level, so the snapshot size may not match what the filesystem reports.

The filesystem may not immediately mark blocks as unused, which makes the block storage system include them in snapshots even though they don’t contain data.

You can trim or discard unused blocks to make volume snapshots smaller and therefore less expensive. Configuring periodic fstrim. You can also mount volumes with the discard option, which helps ensure that the block storage system knows which blocks are used and which are not.