Cloud Drive Mapper works over Web Dav, which is unable to pull in the correct size of the mapped location from SharePoint online. So the size that you see in explorer is the size of the local system drive.
It doesn't mean that the size of the location in SharePoint is affected, for example if its a OneDrive Mapping, the user will have 1tb of space. This is still the same in the mapped drive, however the display will be a false reading.
CDM doesn't download all of the data from SharePoint or OneDrive to the mapped drive when it is authenticated. The data will be there to view and only downloaded once a user opens it. For example if a 20mb word document is opened, then 20mb of temporary space will be taken up on the local system drive, once the file is closed, that space will be released.
We are developing a non Web Dav based version of CDM, this will not be affected by this issue.
If you require any further information then please contact firstname.lastname@example.org