If you use VirtualBox, you might have noticed that the scaled and fullscreen modes are mutually exclusive.
There are several tickets in the bug tracker going back several years ago that deal with this (links appended under references), but it seems that the "scaled and fullscreen"-feature won't show up anytime soon. So until then you can either use a window-manager that offers the ability to make windows fullscreen independently from the application's preferences or use wmctrl if you use e.g. Metacity, Unity (Compiz), Mutter or whatever.
To use the wmctrl-approach, you should first enable the scaled mode in VirtualBox, then switch to a terminal and run this command to flag the window as fullscreen:
wmctrl -r ' - Oracle VM VirtualBox' -b toggle,fullscreen