When you copy and paste an image from a web browser (an image, not the original file), it often puts it in the paste buffer as a png.

So it's not impossible for someone, without realising, to paste a 50MB image in a reply.
With a decent internet connection, you might not even twig.
And if the server isn't set with filesize limits and transcodes, you can end up with a very small number of images taking up a very large amount of space.
Common examples are hi-res paintings/movie posters with grain, or poor quality (but high resolution) photos with lots of sensor noise.