Animal,
I think we're discussing two different features. First, the + and - on the clipboard, governs the physical size of the clipboard on your screen, and doesn't help remove the clutter.
The real "zoom" feature is the Insert/Delete key. This allows you to change the scale of the map, e.g., you can zoom in and show only 1 sector of the map on the full clipboard. That completely removes the symbol congestion which you're complaining about.
I usually set it to show only my current sector and a little bit of the adjacent sectors, myself.
Cheers.