Yup, exactly.

That screen shot is characterized by large blocks of flat single colors with no color variation within the block. For instance the white areas are 255/255/255 all the way across, not one data bit is different. Formats like GIF and PNG eat that kind of data for breakfast.

You would probably get similar results by saving it as an uncompressed bitmap and zipping it. Which would be similar to what PNG is doing to begin with, under the covers.

We could get into an even more detailed discussion of types of data compression... run length encoding, Lempel-Ziv-Welch, the various lossy methods, etc, but the shorthand is "flat color computer screen shots"=PNG lossless, "photos or complex color variation"=JPG Lossy, will get you the best compression. The Wikipedia article on PNG is good at explaining.

If only, when they made PNG, they could have made it a truly universal file format:
- The option for lossy or lossless compression, so it could replace JPG instead of supplant it.
- The option for palletized images and animations like GIF.

If they had done that, I think we'd all be using PNG for everything now. Ah well.