This is one example where "zopfli" or other optimized zlib compressors don't help, the input data is too simple. "oxipng" (my current preferred png optimizer)

This is one example where "zopfli" or other optimized zlib compressors don't help, the input data is too simple. "oxipng" (my current preferred png optimizer) bring a plain 256x256 image created with imagemagick down to 179 bytes, as long as you tell it to strip all metadata objects. Interlacing doesn't make any difference, it's the same size with it on or off.