You may find it helps to resize your partition before you backup.
http://gparted.org/display-doc.php?name=help-manual#gparted-resize-partition

This way it will always fit back onto the same size SD – you just resize when you image it back using the standard resize card tool in raspi-config.

More info here:
http://elinux.org/RPi_Resize_Flash_Partitions
http://www.raspberrypi.org/forums/viewtopic.php?p=190835#p190835