Is Centralization Confounded with Growth?

There are certain institutions that it makes sense to centralize.  Groups that, more or less, provide a service that many people will always need in the same way.  The extent to which these things are true is the extent to which one's service should be large, impersonal, efficient, streamlined, 'lean', cookie-cutter, etc.  It also makes sense to take measures to make such a service permanent and keep it the same.

Other institutions accomplish their goals better if they are decentralized and (therefore) more or less independant of each other.  The services provided by such groups are likely to be specific to a small demographic or a constantly changing one.  Such a group will be more sensitive to changes in taste, even ultimately superficial ones, but it may be valuable to such a group to be able to change quickly to be able to accomodate such constant unpredicatable changes in their market.  

The tendency for any successful social institution is to grow with time and also to centralize with time, but it is important to distinguish between these two, which are thoroughly confounded.  Those groups which provide their service better in a decentralized, local manner will obviously benefit from growth, but will not benefit from centralization (by definition).  What are some models of organizations that grow without centralizing?  Terrorists and protesters are the first that come to mind, but these aren't good examples because, presumably, both experience external pressures that prevent them from centralizing.  Are there any examples of groups that have, through their own policies, stayed decentralized (fine scale) organizations.  Could there be some pressure inherent in social groups that confounds growth and centralization?