The size of raw images produced by the Whonix build process can be significantly reduced by zeroing their remaining unused space. As a result, the size of VirtualBox (.ova) and KVM (.qcow2) distributed images is greatly reduced too, shortening the download process and making their distribution easier. As a side effect, importation of ova images in VirtualBox is also much faster.
The package zerofree (available in the free Debian repositories) can be conveniently used to perform this task against the raw images (as block devices). Zeroing should occur before the conversion to .qcow2 or .vdi formats.