I have found the cause of giant snapshot:
There were 10Gb .tgz file in one of the directories. I used du -sh /* command to find it.
I suppose this file is an exported snapshot that was made by a previous administrator.
I deleted it and the snapshot was the expected size (about 5 Gb).
As I understand /mnt/backup is some sort of system directory, which is used to perform backups or snapshots by operating system. There are also files in this directory on another gateway that functions normally.
There is another strange thing, which I cannot quite understand:
Directory for configuration files is \var\log\CPsuite-R77\fw1\conf instead of \var\opt\CPsuite-R77\fw1\conf
Therefore backup is about 40 Mb size, while backups at other appliances are 80-90 Mb.
The differences is \var\opt\CPsuite-R77\fw1\conf directory, which is located in \var\log and doesn't included in the backup.
Is there any environment variable for \var\opt\CPsuite-R77\fw1\conf which I should check?
P.S. I deal with old appliances (UTM-1), for which support is already ended, so I cannot involve TAC. I have to maintain this appliances for some time before replacement.