What are the gigantic *.CFS files all about in Eclipse workspace plugin folder?

Indeed, it's Nexus indexes. You can disable partially or totally Nexus indexes retrieval by right clicking on repositories in the Maven repositories view and checking/uncheking Disable Index Details/Enable Minimum Index/Full Index Enabled (options with M2Eclipse 0.12).

Hereunder a screenshot:

As stacker points out, each of these seem to be a Lucene segment file, with Compound File format. If you want to see what is inside, you can open them with Luke.

Every repository that m2eclipse uses is indexed by the Nexus Indexer. If m2eclipse is using a remote repository, it will download a Nexus index from the remote repository. If m2eclipse is managing a local repository (local or workspace) it will use the open source Nexus indexer to create and maintain a local index. This index is what allows you to quickly search for and locate dependencies by artifactId, groupId, version, or classname.

http://books.sonatype.com/m2eclipse-book/reference/repository-sect-repo-view.html#d5e1169

You might also consider unchecking the checkbox (under Window menu, Preferences, Maven):