After it is moved to another host, since it is a sparse file, the qcow2 takes the full space, e.g. if it was dynamically set to 100GB, it will take 100GB, even if only 50 GB were actually used.
I could get back more than 20 GB from my Win7 VM (from a 50 GB file) using this:
sudo qemu-img convert -O qcow2 win7.qcow2 win7_small.qcow2
Enjoy!
Source: http://www.tuxfixer.com/how-to-shrink-openstack-qcow2-image-with-qemu-img/
Just Some Techie Notes! 