As time progresses, mdl_backup_log grows and grows... unless i EMPTY it by using SQL : TRUNCATE TABLE `mdl_backup_log`

If all backups goes well, It seems to use unnecessary RAM or Disk space.

How about adding a log purging function to the cron.php script that removes older then one month log entries?
The "keep dates range" can be adjusted from the admin menu.

KeepDatesRange = 30; (days)

DELETE * FROM `mdl_backup_log` WHERE from_unixtime(time) > DATE_SUB(now(),INTERVAL KeepDatesRange DAY)