If you copy your personal image of an OpenELEC Raspberry Pi installation e.g. to another SD card which is larger, you might want to use the whole available space for your /storage partition. But this is not done automatically.
There are some tutorials using resize2fs and other tools, but OpenELEC provides a simpler way:
Connect via SSH / Putty, login and type:
According to this OE script, it does not work any more, unfortunately.
The next best way now seems to be Gparted… or this script (untested!), or make a backup and reinstall.