Strange size of backups

Hello. Community help is needed. I use Linux UrBackup 2.4.13 serer, on the clients UrBackup Client (No tray) 2.4.11. File storage is implemented on ThecusN8800, the folder urbackups is created. This folder is mounted via fstab: “//ip/urbackups /mnt/urbackups cifs credentials=/cred,iocharset=utf8,uid=user,gid=user,vers=2.0 0 0”. The problem is that the size of the backup files is very different. In the web interface, the statistics section shows that the entire volume of occupied copies is 80Gb, when the actual one is 770 Gb. What could be the problem, I ask experienced people to help.