For a presentation I've been putting together, I used Edwin Groothius's nifty pkg_tree tool to generate recursive dependency lists for x11/kdebase3 and x11/gnome2-lite. Both those metaports are the smallest package groups needed to obtain a respectively functional desktop environment.
The kdebase3 recursive listing was about 1.5 MB. Many
thanks to Koop Mast for providing it.
kdebase3 dependency flowchart
The gnome2-lite recursive listing was about 300 MB.
Keep in mind that this is not the full GNOME
desktop (x11/gnome2). This doesn't even include the standard
applications. You can't do much with a gnome2-lite desktop on
gnome2-lite dependency flowchart
So, the next time you install a GNOME application and wonder why it takes an absurd amount of time to register the port, think of the flowchart and it'll make much more sense.