New Super Mario Bros. Wii won't fill screen

[cpotisch]

I can't get New Super Mario Bros. Wii to fill my screen. I've got my PC hooked up to a 50" widescreen 4K TV, and for some reason, NSMB Wii is the only game which won't fill the screen. The game appears to have the exact right aspect ratio for a flat screen tv, but it's only using up maybe 60% of the screen. Any ideas?

Thanks!