Another thing might be that having too many committers makes it less valuable to
become a committer and therefore discourage new people.
In my experience, quite the opposite is true: Having many committers is
beneficial for a project and makes life easier for everybody. There's
more eyes to look at code contributions, more people answer questions
and it also makes it easier for other committers to take time outs from
the project (which might be necessary if they change jobs, have kids, etc)
I think the important thing to look at is not the number of committers,
but the culture and processes in a project.