I am using Oracle VM VirtualBox Manager to run VMs on my machine. I noticed that while running those instances I was not able make it Full screen.
You can see from the below screen shot the unused grey area around the VM instance screen.
In order to locate the cause I went tp ScreenResolution section of VM instance and noticed it was showing 1233x864 which should be 1920x1080
In order to fix that I went to following path "C:\Program Files\Oracle\VirtualBox" via command prompt and ran the following command
VBoxManage.exe setextradata "Your VHD name here" CustomVideoMode1 1920x1080x32
Checked the "Enable 3D and 2D acceleration" check boxes in Display settings of VM instance.
After doing that I selected the "View >> Auto-resize Guest Display" option.
And, you can see it changed to FullScreen