Some digging revealed that "dangling" images are the culprit.
While being a little simplistic, these are images that are no longer associated with any running container.
I dare you to go onto your docker host (even boot2docker) and do this:
docker images -q --filter "dangling=true"
My guess is that you'll see hundreds (or thousands if you've been busy) of those.
Here's the trick to getting rid of those large incontinent beasts that are leaving great piles of steaming images in your host:
docker images -q --filter "dangling=true" | xargs docker rmi
You may see a few false positives, but "hey!" at least the trash has been taken out.
Might be worth cronning it...