Ubuntu thinking directory size is larger than it is duting copy

I have just mounted a 1TB hdd (using Disks utility) at media/username/drivename. It contains a folder folder that is ~140GB in size as per

  1. df command applied to the media/username/drivename/ which shows 138299660KB used and 773225836KB available.

  2. right clicking the folder and seeing its properties which shows 791.8Gb free with folder being 140.7GB

  3. using Disk Usage Analyzer utility which shows 841.8GB of 983.4GB available

Inspite of all this when I try to copy a file from the hdd to Desktop(in the boot drive ssd with ~170gb available) the system says that there isn’t enough memory in ssd

273.2 GB more space is required to copy to the destination.

This message is displayed when the “Preparing” progress bar(at the start of copy) is showing

Preparing to copy 2859633 files(438.5GB)

i.e its gauging folder‘s size to be ~711Gb.

Can anyone explain whats happening?

Env:

Ubuntu 18.04.2 LTS
256Gb sdd, available ~251GB Total(As per Disk Usage Analyzer)
1TB hdd, available ~983GB Total(As per Disk Usage Analyzer)

PS:
The ultimate goal is to make a 700Gb partition on the hdd to install CentOS and then copy folder back into the remaining space where it will keep getting used by the backup software running on ssd’s Ubuntu.