I founded this interesting behaviour then trying to collect Explorer by nfs.
If you want to know size of automounted filesystem you can see unexpected results:
root@serv # df -h /net/hostname/sharename Filesystem size used avail capacity Mounted on -hosts 0K 0K 0K 0% /net/hostname/sharename
but if you do:
root@serv # df -h /net/hostname/sharename/. Filesystem size used avail capacity Mounted on hostname:/sharename 128G 75G 53G 59% /net/hostname/sharename