The PNG files seem to be very poorly compressed.

  $ oxipng -o max --strip all -avZ --fast acius.png
  Processing: acius.png
      2304x2880 pixels, PNG format
      8-bit Indexed (2 colors), non-interlaced
      IDAT size = 84251 bytes
      File size = 84326 bytes
  Transformed image to 1-bit Indexed (2 colors), non-interlaced
  Trying filter None with zopfli, zi = 15
  Found better result:
      zopfli, zi = 15, f = None
      IDAT size = 24466 bytes (59785 bytes decrease)
      file size = 24541 bytes (59785 bytes = 70.90% decrease)
  24541 bytes (70.90% smaller): acius.png