The Bonsai Framework concept of Cloud Density is to reduce cost using a combination of isolation technology and techniques like using multiple instances.
Some examples of enabling isolation technology are Container (FreeBSD Jail, LXC, Docker, Garden) and platforms (Cloud Foundry, Apache Mesos).
In best practice, the output of Cloud Density should reduced cost due to less Cloud virtual machines. At the same time, the system should match the service level of a standard larger cloud.