Display resolution not recognized on 5K iMac

The 5K iMac has a resolution of 5120x2880. However, the game is running at 2560x1440. This is technically the resolution the computer is running at, but with a 2x scale. So the game should be running at the full 5k (I'm assuming it would make edges look much smoother) or just indicate 2560x1440@2x.