The OS does not take up that 27 GB HD space. However, that space is being consumed by the support and data files for some of the applications that were bundled with your new computer. iTunes is an example of an application that will go a long way toward solving your problem of having too much free HD space. If you have the complete iLife suite, .... Well, you get the idea.
it seem something is wrong. there SHOULD be more free space (about 20gb at least) even with all the preinstalled applications.
If you dont want to do your own music you could get rid of garageband. there is a big folder in root/library/application support called Garageband, and there is a big folder in root/library/audio/apple loop/apple called Apple loops for garageband. together they take up about 3gb. do a spotlight search for garageband if you have trouble finding them.
you can download this application and try to pinpoint what folders are the largest: http://www.macupdate.com/info.php/id/13006
I just did a clean install when I upgraded to Tiger, then reinstalled my apps (4.18GB App folder) and other files (2.4GB of music, 1GB of pics, etc.). With everything back on I'm only at 13.27GB used of the 37.14GB capacity. That would mean Tiger sucks up about 5GB or 6GB for me.
It might be that you installed the printer driver and all of the language packs which takes around 3 - 4GB of hd space. The setting to disable these optional components is easily missed during the OSX setup . Fortunally Apple included an app on the OSX 10.4 install CD (I believe its called "Optional Installs") - which enables you to remove additional printer driver and language packs.