Why is my snapshot's size different from reported disk usage?

Snapshots of Droplets 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.

You cannot currently download DigitalOcean backups or snapshots, but you can use third-party tools to save your data locally.
You cannot downsize a Droplet from a snapshot. Data is not always stored sequentially in memory, so reducing the size of a disk can result in data loss or corruption.
You can recover your Droplet if you have taken a snapshot of the Droplet or signed up for automated backups.