It's basically about whether applications are DPI-aware or not. Applications that are get the full 2560x1600 resolution. Applications that don't, don't; instea

It's basically about whether applications are DPI-aware or not. Applications that are get the full 2560x1600 resolution. Applications that don't, don't; instead, they get 1280x800 by default, for an 1:2 upscaling. If you want, you can change that number, of course at a performance cost. People are complaining because even most of the apps that are DPI-aware (Chrome, etc.) don't offer a way to change the physical size of their chrome, but that isn't really Apple's fault.