I now use rdiff-backup in this script to keep 32 days of incremental backups. Beside this the script do a monthly full archive of the website in bzip2 format. This new strategy has reduced the total size of my backups from 64 GB to 6.7 GB. Roughly 90% of free space gain thanks to rdiff-backup! If rdiff-backup is so efficient in my case, this is due to the existence on my websites of large files that are rarely modified (Mp3s, Flacs, RPMs, images, etc…).