pleskbackup : select compression and verify backup

[LoïcM]

Hi,
i'm running plesk 11.5 on ubuntu 12.04.
I run the following command to backup my server :

/opt/psa/bin/pleskbackup server --output-file=/var/home/backup/sauvegarde.tgz

but the resulting file doesnt seems to be compressed ( gunzip -t says : not in gzip format ).
Can we compress the result file from pleskbackup ?

Now, I've found that my backups have always the same size around 4GB, and when I want to extract files using 7zip it says "There are data after end of archive" , I tried to verify with tar tvfW sauvegarde.tgz

but I see many warnings like :
"tar: clients/clement/backup_clement_info_1408090015.xml: Warning: Cannot stat: No such file or directory"

So are my backups valid ?
Thank you if you can help