> A major point of containerization is more efficient resource utilization

It's actually "more efficient resource utilization per instance". Perhaps, with increased capability comes increased utilization. While before one would run only one or two VMs and try to run all their apps within that, now one can run many containers.

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That, and shareable memory across host and guests.

But I guess that works only if the host and guest run the same kernel. Docker and vagrant on OS X still have to dedicate some RAM to the Linux VM running as docker host, and I don't know how efficient other facets of docker on OS X are; but on Linux, I find containerization's memory usage almost at par with running the apps directly on the host.