There are very clearly differences in how organizations have to be operated as they grow. And one byproduct of that is a person who may be a fantastic fit for an organization of size x may no longer be a good fit for an organization of size 5x or 10x.
Of course, people leave organizations for many reasons. But a ton of people who like being in a small (however you care to define) growing organization don't have as good a place in a larger, more process-heavy one with more narrowly-defined roles and responsibilities.
