Finally, from a volume details page, you can manually select two snapshots and compute the delta.
Its Volume Snapshot Details page that too when view at an individual volume level and not at filer/group levels.
I don't know if that's available from the CLI.
You can use the dfpm dataset snapshot reclaimable cli.Its works for even for volumes which are not part of a dataset.
The only cavet is that you will need Provisioning Manager license.
[root@lnx]# dfpm dataset snapshot reclaimable help
NAME
reclaimable -- compute space reclaimable from deleting snapshots in a volume
SYNOPSIS
dfpm dataset snapshot reclaimable <volume-name-or-id> <snapshot-name-or-id>
[ <snapshot-name-or-id> ... ]
DESCRIPTION
Compute space that can be reclaimed if the specified
set of snapshots are deleted from the given volume.
[root@lnx]#
Regards
adai