Make an size optimized black and white png

 Is there anyway you could help me with a quick question? I want to figure out how to do a single bit color, just black and white png, so that it can be smaller.  I tried adjusting the color-depth, but the file is always:` PNG image data, 200 x 200, 8-bit colormap, non-interlaced` and the same size, even if I pick 1,2,4,8 for the color depth.

