S-video is not an HD signal. At its best, it can only do 720x125. Above 125-140 horizontal lines, it can not properly display the color information at NTSC spec (as there is not enough bandwidth in the 3.58MHz signal rate used in the S-video standard). Now, you "can" encode the signal to get more resolution out of it, but to do so, the display would need to also know how to read the signal at a faster rate than the standard, or how to read a different color encoding which limits the color space used in the image to allow for an increase in the scan lines the data represents. Again, both of which require the encoding end of the video and the decoding end to support those alternative methods, and in essence, just use the wires to implement a completely new encoding scheme on the cable.
Otherwise, all you are displaying is a horizontal line doubled 720x480 interlaced video (as the 120 horizontal color information lines double to 240, which is then used to create the interlaced frames) upscaled to your TV's native resolution (or in the case of setting an output resolution other than 720x480i on a PC, a signal downscaled to 720x480i and then upscaled to the TV native resolution).
