My Leopard TextEdit is exactly 15,058,042 bytes. I've installed with the default language settings which means most applications include a basic set of languages built in. I could see how someone in my situation would see a huge decrease in OS install size by eliminating most languages, but I'm still perplexed at your 12GB savings.

I suppose it could be be tightening up compression on images files (and other resources) and better (more optimized) compilation of the binaries.