The very first link in the article leads to the previous post [1] that does mention CoreOS.

[1] http://blog.docker.com/2015/02/the-new-minimalist-operating-...

CoreOS is not huge but is also much bigger than these systems. The virtualbox image for example is 113MB compressed and 358MB uncompressed.

http://alpha.release.core-os.net/amd64-usr/current/coreos_pr...

I think that's only because they don't see CoreOS as "minimal" in the same sense as Rancher and boot2docker.

CoreOS isn't intended to be the base image of a docker container, it's meant to be the host for docker containers.