When i first installed Ubuntu inside a VirtualBoxVM on my laptop, i noticed that it didnt make proper use of the widescreen. The display was a small square box in the middle of the scree which kinda bothered me. To overcome this, just install the guest additions. To do so, press HOST + D. There you go. The auto-installer will take care of the rest.
Sit back and relax while the process completes, and then restart the VM.