Example of how to resize and recompress a disk image for openstack

gistfile1.txt

$ openstack --os-identity-api-version=3 image save --file <filename> <image name>
$ virt-df -h -a <filename> # shows used/free disk space
$ guestfish -a <filename> <<EOF
run
list-filesystems # informational only
e2fsck-f /dev/sda1
resize2fs-size /dev/sda1 <size>G
exit
EOF
$ qemu-img create -f qcow2 <filename>.qcow2 <size>.2G # Add 200MB as padding
$ virt-resize --resize /dev/sda1=<size>G --no-extra-partition <filename> <filename>.qcow2
$ qemu-img convert -O qcow2 -c <filename>.qcow2 <filename>.qcow2c
$ virt-df -h -a <filename>.qcow2c # validate changes
$ openstack --os-identity-api-version=3 image create --disk-format=qcow2 --min-disk=<size+1>G --file <filename>.qcow2c <image name>

Have you found that using the -c option actually results in a much smaller disk? I've done some internal testing and found conversions move super fast without the -c flag, but the resulting file is definitely larger. What format is it being compressed in?