I have definitely seen that happen with the PHP blog engine that runs my blog, either with the GD image functions or with the imagemagick integration. Probably

I have definitely seen that happen with the PHP blog engine that runs my blog, either with the GD image functions or with the imagemagick integration. Probably something like: Take an optimized image, like an indexed png, load it as raw image data, scale it down, save it in the same format but not optimized. The Wordpress code looks similar.