I'm not a Virtual Box expert, but this question occurred to me:
What is the resolution of your monitor and what resolution does the guest OS think it is using?
It could be that the guest OS is maxed out (unless you try tweaking it for your monitor). I know that when I installed guest additions for Windows XP, my Virtual Box went from 800x600 to 1028x764, but my monitor is 1280x960. Same with Fedora and Arch.