I did think about the extra overhead of 64bit binaries, but for some reason assumed the combined effect on this new OS would be smaller. That said, when you can compare that the executable portion of an individual application has not only remained at a similar footprint, but may have actually grown, where does the majority of the savings actually come from?

Have they left off large amounts of resources from the installed applications? Such as localized interface, txt and image files? This could also make for a faster install with less files to write. A huge savings can be had here, as for some applications, resources account for the overwhelming majority of its installation footprint.

I knew this OS would be Intel-only long before knowing what it would be called. Quite a few months before Leopard itself was released. It's great for Intel users, but really a shame in its timing for PPC systems. Apple still needs to support some of them until at least through 2011 (that's a 5-year support lifecycle), so don't expect that Leopard updates will stop.