In non-technical terms, there are only so many pixels to go around on an LCD screen. In a smaller screen, they are packed more tightly than a big screen. That's why Sony's 42" LCD looks better than their 70". Of course the 70" does carry a pretty awesome "holy s**t" factor.

This is a serious comment, but the explanation I got was long and technical, likely causing me serious brain damage. The above nets it out.