After successfully extending a partition on a VM, Disk Management shows the correct partition size but Explorer still shows the old size, even after a reboot.
The reason for this is that while the partition has been extended to the new size, the filesystem itself has not. Normally, Disk Management should first extend the partition, then the filesystem, but for some reason, the second part did not happen.
To resolve, perform an extension of the volume using diskpart.
Open an elevated command prompt and start diskpart (type diskpart and press Enter).
List all volumes to find out which volume number corresponds with the drive you need to resize: DISKPART> list volume