What you are talking about is a custom initial installation of Mac OS X. With that, you can choose to install printer drivers, languages, fonts, etc.
The basic function of these applications is to slim down to the parts your specific system needs to run it.
For example, there are PPC Macs (G3, G4, G5 64-bit) and Intel Macs (CoreDue, Core2Duo, etc). There are things called "Universal Binaries," applications that can run on both Intel and PPC Macs using the same ".app" . Each application ".app" is actually a folder containing code to run the program for each platform. Those with Dell Mini 9s are using an Intel Atom processor. So a 50 MB application, for instance, can be slimmed down to maybe 30 MBs if you remove the code for PPC (32 and 64-bit) and maybe Intel 64-bit.
Then there are language translations in the program; for instance Spanish, English, or Japanese Help files, menu items, etc. For most people, only a single language is needed to operate the application. Strip down the ".app" even more to the language you need. Mac OS X might do this on the initial set up, I'm not 100% sure, but other applications you download or install may have the multiple language support.
Hope that helps.
Edit: Okay, when I opened the thread I could've sworn there were no replies. Maybe a bug that only showed the original post? The previous two posts pretty much answered it in a less verbose manner.