Disable the paging file (Settings → System → About → Advanced system settings → Advanced → Performance → Settings… → Advanced → Virtual memory → Change…), clean all temporary files (C: → Properties → Disk Cleanup), defrag (defrag C: /FreespaceConsolidate /Verbose), and then zero out free space with SDelete (sdelete -z C:).
Then, compact the disk image on the host:
vboxmanage modifymedium /path/to/image.vdi --compactRemember to turn the paging file back on after compacting the drive!