It is a matter of dots.
My little 4" display has about 320x180 dots. (pixels)
The minimum computer display is 640x480, so about half of the dots aren't being displayed.
It just so happens that NTSC is about the same resolution. (320ish x 180ish)
So really a computer display will always look worse on a Composite input screen (rca plugs) than a VGA screen.
Now as the screens get larger, you have more dots to play with, so you can see it from farther away, but still the resolution is the same.
After playing with this 4" screen, I think that the 6.5 or better displays are the only way to go.