I bet they check in (or have checked in at some point) 3rd party libraries, jar files, generated files, etc. I battle this every day at $dayjob and we have a multi GB subversion repository with a separate one for the 3rd party binaries. Svn handles this a bit better than the DVCSes, so just checking out the HEAD is smaller than the full history you get in git/hg, and you can clean up some crud a bit. It just lives on the central server, not in everyone's working copy.
You can clone with a single revision in git. Still ends up being around 2X the size I believe since you have the objects stored for the single revision as well as the working tree.
