In my expererience, the shrinking or growing of any congregration is mostly due to either geographic migration of existing JWs or the ever-present "congregation shoppers" who tend to flock to the most popular congregation of the moment. The latter seems to occur in waves-- either lots of young people all want to hang out together, or pioneers move into a particular hall because there's lots of support, so the numbers bump up over the course of a few years, level off, then slowly trend downward as people move on to seek greener pastures.
R_O