GTX 1080 can't handle 512MB array on Windows

So I have a GTX 1080 and GTX 745 in a Dell XPS running Windows 10. The 1080 isn’t plugged into a monitor and I’ve set the other card as my primary display in the BIOS. Nonetheless, when I run gpuDevice in Matlab, over 1GB of memory is consumed–

                  Name: 'GeForce GTX 1080'
                 Index: 1

TotalMemory: 8.5899e+09
AvailableMemory: 7.0649e+09

and although it can handle a 392MB array passed in through Matlab (2 7000x7000 double arrays, it chokes unrecoverably on 8000x8000. This seems low to me. Is there anything more I can do to ensure the graphics card is being used only for computation? What is using the 1.5GB of RAM? I see this even before I’ve done anything else with the card after booting.