Ultra compression like Matt Hahoney has worked on and keeping the distribution to the source code, rather than built packages for all supported architectures, could probably squeeze this down onto 1 DVD. Debian doesn't really support truly antique computers any more, pretty much everything out there that it runs on now can compile everything it needs. Binary packages made a lot more sense when you were running Linux on a Zaurus with 16-32MB RAM which isn't enough to compile large programs or the kernel.