IIRC the transistor on each pixel colour in the display blocks a fixed amount of light, so the increased pixel density means less area for light to pass through

IIRC the transistor on each pixel colour in the display blocks a fixed amount of light, so the increased pixel density means less area for light to pass through, so a brighter backlight is required to achieve the same light output. (It's possible of course that these high-DPI displays use a process technology with smaller transistors, anyone know?)