Huh that's weird, my Firefox definitely reported that. I also checked the other image, that size matched perfectly with the claim in the text. My first thought is http-level compression, but that shouldn't be that effective on an average image.
(I'm not at my computer anymore so I can't check what you suggested right now.)
Incorrect, it is 36.5K:
When you tried reproducing the results using GIMP, were you creating similarly constrained indexed-color/paletted images?You likely won't get the same compression levels with rgb pixels, you can use `pnginfo` to compare the file properties: