How to compress a qcow2 image?

Try first:

qemu-img convert -c -f qcow2 -O qcow2 filename output_filename

If it doesn't reduce it, try this:

qemu-img convert -c -f qcow2 -O raw filename output_filename
qemu-img convert -c -f raw -O qcow2 output_filename output_filename2