It is not easy to see the size of the folders in the Google Drive, so I thought that downloading all of them on the local drive would let me see clearly contents and sizes.
After doing that I see that the size of the downloaded folder is 1.47GB, while Google storage shows a total of 4,89 GB for Google Drive.
Here is the "Properties" window of that folder:
That is confirmed in Windows explorer after installing the Google Drive app (thus adding the drive to Windows Explorer):
and by investigating with WinDirStat (as said in this great answer):
Why is that?
(I don't think I have versions of the same files taking up space, and even if there are some they must be negligible.)