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A PNG is pretty much a zlib compressed bitmap.

That's not true. PNG has some filters that pre-compress the image before sending it to zlib.

I'd say that still falls under the realm of being "pretty much a zlib compressed bitmap", no?

No, I wouldn't say that. Those filters make a huge difference to the compression ratio!

The png optimizers you see (optipng, pngcrush, etc)? What they do is carefully pick the optimum filters to get the best file size. So I'd say those filters were the most important part of the compression.

One of the few truly interesting articles on Smashing Magazine about optimizing pngs for their filters. It shows that yes, the filters are really a huge part of how pngs get compressed:

http://www.smashingmagazine.com/2009/07/15/clever-png-optimi...

You are correct. I almost qualified the statement as such, but figured "pretty much" was good enough to get my point across in a hurry. My point being that zlib compressed 24-bit RGB values in a PNG is normal. It's a different approach than lossy DCT quantization used in JPEG.

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