I just started to test out KVM version; both the downloaded one and one I compiled from source (the one I compiled from source was missing a kernel though, I will post that in new topic).
My host system I downloaded the ‘Whonix-Gateway-8.tar.gz’ file is Ubuntu Server 14.04 running btrfs file system. When I extract the file it is 100G. Also, when I built the file from source, the final qcow2 image in ‘whonix_binary’ was also 100G.
In order to be able to untar the qcow2 file at the expected size of 2.6G and not 100G I would suggest you add one more step to the build process as shown in the example below.
Running qemu-img convert on the freshly created qcow2 image will compress it and allow it to be extracted from a tar at the 2.6G file size instead of the 100G file size. The qcow2 image itself is reduced from 100G to 2.6G as also shown and the tar.gz image is still 1.2G. As you will note we are converting a qcow2 to a qcow2
qemu-img convert -O qcow2 Whonix-Gateway-8.qcow2 Whonix-Gateway-8-downloaded.qcow2
# ls -l > -rw-r--r-- 1 root root 107390828544 Feb 20 18:48 Whonix-Gateway-8.qcow2 > -rw-r--r-- 1 root root 1211742737 Feb 26 07:55 Whonix-Gateway-8.tar.gz # qemu-img convert -O qcow2 Whonix-Gateway-8.qcow2 Whonix-Gateway-8-downloaded.qcow2 > -rw-r--r-- 1 root root 2233729024 Mar 23 17:19 Whonix-Gateway-8-downloaded.qcow2 > -rw-r--r-- 1 root root 107390828544 Feb 20 18:48 Whonix-Gateway-8.qcow2 > -rw-r--r-- 1 root root 1211742737 Feb 26 07:55 Whonix-Gateway-8.tar.gz