Deleting files in Windows only makes changes to the MFT, logically deleting space. In order to free up that space physically the OS will need to tell the storage which blocks are no longer used and can be freed up (assuming you use thin provisioning such as Nimble does by default). This function is called TRIM or UNMAP and as far as I know this is supported in Windows 2012 for example by using the "optimize-volume" command with the -retrim parameter. Older Windows OS'es could do the same by using the sdelete tool with the -z option.
You do need to make sure the files are permanently deleted (not in the recycle bin).