So I did some testing this evening: someFile.jpg is an image file that was synced to the computer via iCloud Drive:
==========
Used space : 958,537,990,144
cd ~/Library/Mobile Documents/com~apple~CloudDocs/Temp
ls -l someFile.jpg
907708 2 May 2017 someFile.jpg
du -h someFile.jpg
1.7M someFile.jpg
cp someFile.jpg ~/tmp
Used space : 958,538,326,016
cd ~/tmp
ls -l someFile.jpg
907708 15 Aug 22:39 someFile.jpg
du -h someFile.jpg
888K someFile.jpg
mv someFile.jpg ~/Library/Mobile\ Documents/com~apple~CloudDocs/Temp/someFile2.jpg
Used space : 958,544,031,744
cd ~/Library/Mobile Documents/com~apple~CloudDocs/Temp
ls -l someFile*
907708 2 May 2017 someFile.jpg
907708 15 Aug 22:39 someFile2.jpg
du -h someFile*
1.7M someFile.jpg
888K someFile2.jpg
========
The file that was synced seems to exhibit this behaviour (disk used is about double file size). If itâs copied to another directory on the same filesystem, it behaves as exprected (disk used is about the same as file size). If moved back to the iCloud Drive directory tree (still the same filesystem on this computer though), the file retains its expected behaviour.
Also: Checking used space via Disk Utility before and after a full iCloud Drive sync indicates that the disk used is about double the expected amount (and double what it is on my other two computers).
This is very strange.