This happens all the time, all over the web. It's because interns or non-webdev staff often don't know they should resize a file before uploading it and attaching it to a post. So they'll upload a JPG right from a camera's memory card and use the WYSIWYG editor to set its dimensions. The worst are when images like this are referenced within broadcast e-mail messages. Luckily you can swap the file out on disk without inconveniencing future readers.

I wrote an image API at my place of work to take a file by name and accept dimensions in the query string. It resizes the image, caches the file on disk, at and serves the smaller file. However, it's tough to integrate that API with every little WYSIWYG editor we have, so sometimes we still serve up large files in this way.

Usually, intentional retina-quality images have plenty of telltale signs (like the @2x naming convention).