Because somtimes Medium is too large.
Small aims to be an easy to use and install web application which provides users with a Medium-esque service, but for individuals and small groups instead of everyone in the world.
In its current state, Small is based off of an older project, Branch. As it's currently exactly the same as Branch (as in, just pulled the source), there are references to the name "Branch" all over. That'll change, just like a good deal of other things.
If you're looking for more information on both how it currently works, and how
it should work in the future, check out the commit log (most commits are
fairly descriptive) from before April, and the ./spec directory, which has a
bunch of design documents.