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:
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.
> On the following pages are a few of our favorites among those already posted. They and other artifacts can be seen in higher resolution in NIST's archive. If you know what any of them are, let NIST know.
To Wired's credit - this is merely a news article that links to the relevant NIST site.