You can grow too fast.
If you’re just getting started and trying to build the base, you might miss a key step.
If you only have limited manpower to support the community, growth can hurt.
If you’re juggling a job with multiple priorities can you handle 1000 new messages per day?
If you have a mature, happy, and highly-engaged community, will an influx of strangers be a good idea?
We need to change the mindset that growth is always good. It's not. If growth hurts what you have already, or if it becomes impossible to manage, or if your community is perfectly fine as it is, then growth can be bad.
When you grow, the level of engagement (activity) per member can decline. That can lead to less ROI per member, and thus a less valuable community. If you grow the community to a size you lack the manpower to manage, you're creating problems for yourself. This is just as big a problem as a lack of growth.
Growth is just one channel to have a more valuable community. The other is to increase the level of activity per member and/or increase the ROI per member.
Fortunately, excess growth is easier to resolve than limited growth. You can remove members, close the community to newcomers, open up a waiting list, halt promotion efforts, switch to an invite-only system, have an application form, remove the bad members, or simply make the process of joining the community a little more difficult (e.g. ask for more information on the registration form).
Growth doesn’t necessarily mean a better, more active, or more valuable community. It has side-affects. If you have excess growth, that can be a big (but easily fixable) problem.