for PC-BSD, your example would look like:
pkg_add -r virtualbox-ose
and, you're right about the AppCafe's PBI files being large, but again that's due to all of the included dependencies. if i understand things correctly, after you download and install a PBI, there's behind-the-scenes checking between the installed PBIs, so duplicated dependent libraries are removed. i could be wrong about this, hopefully one of the devs can set us straight on this.
also, don't forget that you may have already installed VirtualBox. remember, if you've already run the pkg_add -r virtualbox-ose command, you've installed it. that being said, i'm not sure exactly *what* you installed, or how to run the application, but it should be there. if you are installing from the AppCafe, you may want to remove the package you've already installed, i.e.
pkg_info | grep virtualbox
see the man pages for pkg_add, pkg_info, pkg_delete for more info. also, the FreeBSD docs are great: http://www.freebsd.org/doc/en_US.ISO...ook/ports.html