Apparently I can't access that file but I'll believe you. Although if the original was jpg as well then it was somewhat unfortunate to save it as jpg again as that would result in additional compression artefacts (lossless jpg exists, but isn't exactly common).

That said it's hard to tell whether it's compression artefacts or some 'smart' sharpening that's responsible for the artefacts. It would need to have been compressed pretty heavily to fit in just 4MB.