Mounting “Haiku” partition from the the TC0 64 bits ISO, Tracker shows: 1.37 GiB total size, 1.02 GiB used, 360.16 MiB free.
And the _sources_ directory has 336.11 MiB of GPL’ed sources.
Edit:
Not much use on the free space for a real ISO (to be burnt into CD/DVD), but it comes in handy when writting the image to pendrives/memory-cards, and having only one image for both cases (anyboot) makes sense considering bandwidth/storage costs.
Not sure why the sources have to be in the image (as they are already available via HaikuDepot/pkgman), when even most Linux distros don’t include sources on their install media.