Because it keeps around all of the images you use as well as the containers after they are stopped so you can resume them. there is a command that auto automatically removes all unused images and containers you can use.

True. But there are situations where things get ~corrupted, and it becomes ~impossible to recover without basically nuking everything. But then, I was running containers with resource limits and long-term storage. So I could have thousands of lightweight "VMs" per physical server.