I bought a LG 27UK650-W monitor that has 4K UHD resolution (3840 x 2160). When I connect it to my MacBook Pro (2019, 16-inch, Big Sur), the fonts are huge. I had to switch from "Default for display" to "Scaled" and move it down a couple of notches. Once I've done that, the System Report under About This Mac shows:
Resolution: 5120 x 2880 (5K/UHD+ - Ultra High Definition Plus)
UI Looks like: 2560 x 1440 @ 30.00Hz
Framebuffer depth: 30-Bit Color (ARGB2101010)
It is listed as LG HDR 4K Display 27-inch (3840 × 2160) under Displays in About This Mac.
By comparison, my work laptop has a Dell U2719DC with 2560×1440 pixels and it works perfectly with the default settings (no scaling).
I tried both a UtechSmart 6 In 1 USB C Hub and a Belkin USB-C to HDMI Adapter (HKQ22ZM) with the same result.
Shouldn't Apple "just work"? I expect to plug in a monitor and have all pixels available and the UI in some sane state in terms of font sizes. What am I missing?
Best Answer
Some things aren't adding up here. You say the monitor is 4K, instead system report indicates some display has a 5K resolution.
However, I think you are misinterpreting the numbers. The scaling option still renders at the highest resolution of your monitor, it just enlarges the UI. What you are losing is space on your monitor, because UI elements will appear bigger, in order to be readable.
Imagine an app opening by default with a resolution of 1600x900. If your monitor only has this resolution, it will fill the whole screen. If you instead have a 4K screen, that app will only take some small space on the monitor. Now if these monitors were the same size physically, the app will appear much smaller on the 4K screen. Now scaling allows you to change the appearance, to match the app's size from the 1600x900 monitor on the 4K screen and will display the app fullscreen. The app ends up physically being the same size, yet rendered and shown at the high-res of the 4K monitor.