The true seed of scale an online community is love. It is better to have 100 users who love your community, than 1 million who just like it. If you want to make your community bigger, this is good, but never forget to keep it loveable.
This perfectly explains that in an online community, you should never scarify quality for quantity. Whatever your purpose is, the scaling should be controlled. By scaling an online community, you should help the community to get closer to its goal. In order to successfully scale a community first, you should think about what your community needs.
If you want to grow your community, you need to have some numbers. You need to know what percentage of your users are active every day or every week? What are they doing? And how are you going to produce more of them?
You don't need all your members to be coming back every single day. But you do want to identify your hardcore members and figure out how to grow that group because that is the core of your community creating most of the value.
Also, if you want to have a big community, you need to be able to break it down into smaller groups that you can control them more easily. All groups will break down in its ability to efficiently self-manage, at a big enough size. So, in order to scale a community which is still healthy, you need to consistently break down that community into subgroups, and provide members with a context that's still relevant to them.