In future there may be other ways to speed up large images by taking advantage of Docker’s layer caching. For instance, if there was a large 10GB base image that was widely used and you added 1GB to it, if that base image was sufficiently popular it might already be on all nodes so only the additional 1GB layer would be downloaded, whereas if you were to build a smaller fully customised 5GB image that might have to be downloaded from scratch.
That’s mostly theoretical at the moment, but there is a discussion on Some steps to explore supporting a "default environment" · Issue #1474 · jupyterhub/mybinder.org-deploy · GitHub about having a “default” binder image.