It depends on the entire partition table (/usr, /usr/local ...), not just your root (/) partition. Installing from ports will keep all sources (even after installation) in /usr/ports/distfiles, but you can delete those with # make distclean or # rm -rf /usr/ports/distfiles/* for example. I don't really know how much space either of those take, but I'm guessing you can do something like # make -C /usr/ports/x11/gnome2 fetch ; du -hs /usr/ports/distfiles/gnome2. Compiled binaries have roughly the same space usage.
