It is lighter because it isn’t built. It’s basically the same as working with it from Subversion. I’ll be updating this document for Git archives and/or clones, but refer to this for now...
Is it possible to grab the latest dev branch from http://github.com/modxcms/revolution/tree/dev to do a fresh install without digging into Git?
The "Download Source" gives me of zip but it comes with the _build dir and seems lighter than the pl release.