Sample images larger than the original

I've noticed on rare occasion that some images have larger sample images (in terms of filesize) than the original image itself. (e.g. post #922455: 327K v. 503K for the sample)

Out of curiosity, why does this happen? Seems like a waste of server resources.

Because samples are created based on resolution.

Then, if the sample would be bigger than the output image, why not just scale the original via HTML image tag size properties?