On 14/06/2013 23:19, Dudley Hanks wrote:
Two files compressed from an original with 16,777,216 pixels and
16,777,216 colours...

One is a .png file, 133,450 bites...
http://www.blind-apertures.ca/pics/16MillionColours.png

The other is a .jpg, 7,416,217 bytes...
http://www.blind-apertures.ca/pics/16MillionColours.jpg

Which one has lost about 3/4 of the original data?

Considering that the target image is inappropriate for JPEG encoding it
has actually done well to achieve a fairly close visual similarity.

Photoshop isn't the best at this by any means. PSPro JPEG manages

5,502,195 bytes and 4007721 colours as a JPEG

and

1,754,469 bytes and 16,377,913 colours as a J2k (lossy)

Moral of the story you need to think about which method of compression
you use on line art and test pieces intended to break JPEG encoders.

--
Regards,
Martin Brown