Backup completes without error but the .tar archive has errors

After waiting a while for backup to complete, it finishes without problem and even uploads the 280mb .tar archive to my remote sftp server. However, when I extract the archive with 7zip it just has a 0kb wp-rss.php file and there is an error:
Headers Error - Warnings: There are some data after the end of the payload data

If I download the local .tar copy from my webserver it has the same result. Attempting to extract the .tar via cPanel file-manager has a bunch of errors like this:

public_html/wp-rss.php
/bin/gtar: Skipping to next header
public_html/images/main.png
/bin/gtar: Skipping to next header
/bin/gtar: Exiting with failure status due to previous errors

Is this a problem with using just .tar and no compression, which I was using in order to save cpu power on my shared webhost, or is this just a misconfiguration with my host/php? Thanks!