Currently there is no way to see how much storage each namespace is consuming. It would be great if this feature was available. There doesn't seem to be a clear way to do this even from the CLI:
root@backup:/mnt/datastore/node03-store# du -h -x -d 1 .
2.9T ./.chunks
350M ./ns
2.9T .
root@backup:/mnt/datastore/node03-store# ll .chunks/
Display all 65536 possibilities? (y or n)
root@backup:/mnt/datastore/node03-store#
root@backup:/mnt/datastore/node03-store# du -h -x -d 1 .
2.9T ./.chunks
350M ./ns
2.9T .
root@backup:/mnt/datastore/node03-store# ll .chunks/
Display all 65536 possibilities? (y or n)
root@backup:/mnt/datastore/node03-store#