Not sure how to do so via flyctl volumes, but you can always flyctl ssh console -s into the VM and du -sH / df -h your way?
# runs the cmd in the nearest vm to the client
fly ssh console -C `du -sH /mount/point/*`
# or ssh into a vm via its 6pn ip
fly ips private
fly ssh console "[6pn:ip:within:these:brackets]"
See: fly ssh console fails with `Error host unavailable: host was not found in DNS` - #5 by ignoramous
(assuming the app’s container has the du or df binary in its path)