Clarify size and virtual size

engine/userguide/storagedriver/imagesandcontainers.md

@@ -72,6 +72,39 @@ writable container layer. Each storage driver handles the implementation
 differently, but all drivers use stackable image layers and the copy-on-write
 (CoW) strategy.
 
+## Container size on disk
+
+To view the approximate size of a running container, you can use the `docker ps`
+command. Two different columns relate to size.
+
+- `size`: the amount of data (on disk) that is used for the writable layer of
+  each container
+
+- `virtual size`: the amount of data used for the read-only image data
+  used by the container. Multiple containers may share some or all read-only
+  image data. Two containers started from the same image share 100% of the
+  read-only data, while two containers with different images which have layers
+  in common share those common layers. Therefore, you can't just total the
+  virtual sizes. This will over-estimate the total disk usage by a potentially
+  non-trivial amount.
+
+The total disk space used by all of the running containers on disk is some
+combination of each container's `size` and the `virtual size` values. If
+multiple containers have exactly the same `virtual size`, they are likely
+started from the same exact image.
+
+This also does not count the following additional ways a container can take up
+disk space:
+
+- Disk space used for log files if you use the `json-file` logging driver. This
+  can be non-trivial if your container generates a large amount of logging data
+  and log rotation is not configured.
+- Volumes and bind mounts used by the container.
+- Disk space used for the container's configuration files, which are typically
+  small.
+- Memory written to disk (if swapping is enabled).
+- Checkpoints, if you're using the experimental checkpoint/restore feature.
+
 ## The copy-on-write (CoW) strategy
 
 Copy-on-write is a strategy of sharing and copying files for maximum efficiency.