That’s how backups work. By default, thumbnails aren’t included in backups. We changed that at least a year ago.
You can configure that behavior with the include_thumbnails_in_backups site setting.
Include generated thumbnails in backups. Disabling this will make backups smaller, but requires a rebake of all posts after a restore.
@eextra I’d recommend creating a file based diff of the content of an old and new backup in order to find out why the backup is so much larger.