I set up the new WD Blue 4TB drive with NTFS with compression enabled because what the hell why not.
Copy all the data from the old HDD to the new one. Data on the old HDD is about 650GB.
... Data on the new HDD after copying says it takes up 1.2TB. The fuck?
Cursory googling reveals a long-standing, well-documented Windows bug where copying data from a non-compressed volume to a compressed NTFS volume makes the OS lock down twice that amount of space until you reboot the system.
Reboot, magically new HDD reports using 650GB.
Just Windows things :)