This really only applies when/if you’ve got small hard disk partitions, &/or are using small dynamically expanding VHDs [Virtual Hard Disks]. A holdover from early days, Windows puts some non-movable, non-removable files partway into the drive, rather than locating them at the start of the disk. Far as I know there’s no way around that. But when you’re doing something like upgrading to a new version of win10, lots more permanent files may be added further in on the disk, and those you can do something about.
Win10’s defrag works better on these permanent files than earlier Windows versions, but it’s not always enough, as with a just updated win10 Insider VM, where it prevented the dynamically expanding VHD from getting any smaller than 15 GB. I tried copying the partition using AOMEI’s app, but that didn’t change anything. Backing up the Windows partition, then restoring it using AOMEI Backupper however did get rid of all those files, moving them to the start of the disk, leaving only the NTFS file tables I mentioned earlier. That allowed the same disk with the same contents to shrink to 11 GB.