It is not recommended to use disk defragmentation if you back up to the Acronis online storage and the disks are of considerable size.

If you run disk defragmentation after creating a disk/partition backup, the succeeding incremental backup will roughly be the size of the full backup. For detailed information please see Acronis Backup Software Creates Large Incremental or Differential Backup Archives.

This may be especially inconvenient if you use the Initial Seeding option to upload the first full backup to the Acronis online storage.