The EXIF/XMP data is highly insightful, I didn't even think about how much space those would take up. I updated the original blog post with the information you provided here, and cited you for it. Hope you don't mind.
I mentioned in comments on your original article: I wasn't able to appreciably beat the BMW or portrait images.
Metadata would help explain this, since as a percentage of file size, it'd be a lot more detrimental on the first image at 45K than the BMW at 575K.
But sure seems there's something else going on with the first image's circular gradient; PS seems to be dithering its blocks in a moire pattern while GIMP is showing strong banding even on files w/o metadata.