WD Passport Go SSD 1 TB - Size of Disk is much higher compared to Size

I am having issue with occupied space. I have copied some stuff from my computer to this drive. In properties, it shows Size 281 GB (3,02,13,47,07,136 bytes) while Size on Disk is shown as 645 GB (6,92,91,99,92,320 bytes).
How come Size on Disk is higher than Size and that too by such a margin. Because of this the 1TB SSD is practically reduced to a 300 GB SSD

The type of the file system is exFAT.
1048576 bytes in each allocation unit.
953862 total allocation units on disk.
249704 allocation units available on disk.

Screenshot (4)