80%+ of the time its due to a misconfiguration of a download client where its winding up saving stuff into the docker.img.
15%+ of the time its due to a misconfiguration of a download client where its winding up saving stuff into the docker.img due to a capitalization error on a folder (ie: /downloads is mapped, but container references /Downloads)
5% of the time its due to excessive logging by a container.
Virtual Sizes AFAIK are only the sizes of the actual container, and nothing else that the container might create within the image. Net result is that its a pretty useless metric, but since the docker system has it available, I make it visible.
You can peruse the Temporary Docker FAQ for some suggestions, or the investigation regarding out of control apps logging "Hi! I'm still running. How are you doing?" every second here:http://lime-technology.com/forum/index.php?topic=45249.0