Onedrive says I have 4GB of storage... But I count 2GB.. Why???

I have files backed up in Onedrive in a folder called 'Personal' which is 4.7GB in size. In that folder also sits 'Documents' folder which is a backup of 'My Documents' folder. It's 4GB in size.

But when I manually count the size, I get 2GB in 'Documents'. I dont get it. Can someone explain this??

...

You are adding up File sizes rather than cluster/block size(size on Disk).
e.g. say a 1 KB Notepad file will take up 4 KB on the drive, assuming 4 KB cluster/block size.
So, a large number of smaller files will take up a larger size on disk.

Asked and answered ... it's OneDrive, after all