This is something of a philosophical issue. On the Mac, you generally don't maximize windows. In general, you try to make the window large enough to show its contents, and no larger. This leaves space for other things (windows/icons/desktop/toolbars) in the remaining space. Only if the window's contents are actually too large to show without taking up the whole screen should a window actually fill the screen.
It was originally sort of a reaction to the old DOS days, when whatever you were doing always took up the entire screen.
Now, to answer your question: If you really must cover every pixel of your desktop, try manually resizing the window (using the lower-righthand corner) to the largest size. Then try quitting and reopening Netscape.
