PNG is often not well compressed unless you've gone out of your way to do so with PNGCRUSH (http://pmt.sourceforge.net/pngcrush/) or something similar
I had an image that pngout got to 218K (221K for optipng). pngnq chopped that thing to 74K. When flipping between the images you could see some pixels change, but only if you looked really hard.
(Running pngout on that image brought it down to 71K. Nice.)