I installed TWRP 3.1.0-0 on my Redmi 3S Prime and tried to make a Nandroid backup. I've never taken Nandroid backup before. I have a linux PC. On my PC, I installed adb and fastoot drivers and used took the backup by typing "adb backup --twrp" in the terminal. My phone showed that the backup size was 9757 mb but the backup.ab file created was only 2.1 GB. Where is the problem? Did I do anything wrong? By the way, I backed up the boot, system and data (excl. storage) partitions.