Compact the PG medium:
vboxmanage modifymedium --compact
Clone, attach and compact the base box:
vboxmanage clonemedium --format VDI /Users/robert/VirtualBox\ VMs/musicbrainz-vm/box-disk1.vdi vboxmanage storageattach --storagectl 'SATAController' --port 0 --type hdd --medium /Users/robert/VirtualBox\ VMs/musicbrainz-vm/box-disk1.vdi vboxmanage modifymedium --compact
Export the VM:
vboxmanage export bead95d7-05b4-4c11-a65a-7952155b0ab1 -o musicbrainz-server-<date, etc>.ova