How to scale the splashscreen on Windows?

In my Windows project, I have included 3 splashscreen images at the sizes specified in the Visual Assets tab of the properties in Visual Studio (1116 x 540, 860 x 420, and 620 x 300). When I run the app on a Windows 8.1 machine, the splashscreen image takes up a tiny part of the centre of the screen, with massive amounts of padding around it.

Is there any way of getting the splashscreen images to take up more of the available screen space?

Thanks,

John H.