Each subpixel needs a transistor (hence TFT - thin film transistor) to drive it, along with the addressing conductors to drive it. I suspect the transistors hav

Each subpixel needs a transistor (hence TFT - thin film transistor) to drive it, along with the addressing conductors to drive it. I suspect the transistors have to have a certain minimum size to be reliable enough, so their relative size to the size of the whole pixel grows with decreasing pixel size. I'm sure they'd make them smaller if they could, as the black areas absorb a lot of precious backlight energy. I also recall reading about progress with making the transistors (which are typically silicon-on-glass) transparent, but I guess that tech isn't ready yet.