The Galaxy S9 has the same 5.8-inch size 3K Quad HD+ OLED Infinity Display and resolution (2960 x 1440) as last year’s Galaxy S8. However, the Galaxy S9 is superior to its predecessor when considering the color accuracy, the brightness, outdoor visibility and reflectivity.
The Galaxy S9 resolution defaults to the lower 2220 x 1080 pixels (FHD+) resolution in order to conserve battery life. If you are constantly charging your device, then I recommend changing your screen resolution to the higher WQHD+ at 2960 x 1440. Or you can reduce the resolution to HD+ at 1480 x 720. You can control this by going to Settings > Display > Screen resolution.