You do realize that all those hundreds of megs worth of libraries also exist on Windows and OS X, they just ship in the box with the OS?
Yep, and on OS X developers use mostly the OS provided libraries while on Linux they pull in all kinds of obscure stuff (how many friggin' XML parsers need to be installed on a system ?)
(which, coincidentally, is why you can have Linux without any GUI at all, but not the other two)
OSX without a GUI exists, it's called Darwin. There have been several implementations even, people just don't care about it.
And, of course, there are plenty of Linux distros that also include all that stuff in the box. If you install a typical desktop app from a package in Ubuntu, it's unlikely to drag in more than a couple smallish libraries - and those are usually where the core functionality lives (i.e. app is just a GUI shell over a library).
Until you have Gnome 3.0.1b453 or whatever and your app wants 3.0.1b567.