this "works as designed". Expanding the volume of a sx80 dont change anything on exsistent lun. You need to expand the lun also.
But, depending on the software you use (SAM or Spheras) this depends if the lun to expand is the last one or not.
Anyhow, expanding the lun does not change the filesystem size the server reports. Last you need an OS component to enlarge the filesystem (i.e. diskpart for Windows or a locigal volume manager for unix systems).
Expanding the lun for Windows system results in diskmanager in that way that a large disk is seen, containing a partition with the old size of your lun and the rest is reported as "unused space".