Many OS X applications have hooks to others, so there's sort of a dependency. One good example is Mail.app and the calendar.
The application packages are only part of what you see—many of them have underlying frameworks in the
/System folder. So, the
.app files are probably just the tip of the iceberg and won't really make a drastic impact.*
The easiest way to save space in OS X is to remove unused languages.
Other tips are mentioned here:
The single most important tip though is to avoid any sort of "cleaning" apps like MacKeeper, CleanMyMac, et cetera.
If you want to go for it, you can force-delete application packages by opening up Terminal.app, and then calling something like:
sudo rm -rf /Applications/iChat.app
However, do this at your own risk. Keep a backup and don't complain the day you run into an error :)
* For example, Messages.app—the successor to iChat—uses around 26 MB. FaceTime is 5.4 MB.