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