No, 10.5 has 64bit Cocoa but no 64bit applications. If you don't believe me, drop to the console and type "otool -f /Applications/Calculator.app/Contents/MacOS/Calculator" — it'll list two architectures. Not four, two. Ditto for everything else supplied with the OS. Then, to complete the proof, hit command+i in the Finder and verify that those same apps are universal. So they at least have PowerPC and Intel code.
Furthermore, the actual executable code tends to be a tiny part of the supplied apps. Mail.app (the largest supplied application) is 298mb in total, of which less than 10mb is executable and the other 279mb is 'Resources' stuff (tiffs, localisations, etc). iTunes (current version) is 26mb executable, 130.7mb resources. iChat is 6.3mb executable, 108.2mb resources. And so on, and so forth.
The entire /System/Library/Frameworks (where Cocoa, etc live) is only 1.7gb. If you reduced Cocoa/etc to zero, you'd still save less than a third of the 6gb. If you chop it by the half you seem to be expecting, even that's only 0.85gb. So far from being "where the 6gb comes from" as you claim.