Here are my results.
du -h Whonix-Gateway-8.tar.gz
du -h --apparent-size Whonix-Gateway-8.tar.gz
du -h Whonix-Gateway-8.qcow2
du -h --apparent-size Whonix-Gateway-8.qcow2
Seems like sparse files to me. At least on my system. Using Debian Wheezy / ext4. Also the "df -h" test indicated this.
I guess the issue isn't with the image itself, but with the file system, host operating system and/or kernel.
Would you be willing to tell us your host operating system? (You don't have to if that is private.) (Would be useful for other users having this issue / reference.)
Whatever your host operating system is, you could try with Debian Wheezy and see if the issue is solved there. If we're already providing sparse images, then there is probably nothing we can do from Whonix side. Except telling you, what host operating systems / kernels support sparse files.
For further debugging of this issue, you could download one of our .ova images. And extract it (or alternatively import it into VirtualBox). The .vmdk image you get to see then should be a sparse image.