As communities evolve, often they slowly grow in size, from an initial SeedPosting stage, gradually acquiring a substantial core group of frequent contributors. This is often desirable, but not always - it is a mistake to confuse size with success. If the size of your community threatens to damage its success, you may wish to LimitGrowth.
CommunityMayNotScale. Certainly a smaller community (or "clique") has a very different feel to a larger community (or "society"), and the success of your site may be dependant on its particular size. This is especially true if you value CommunityOverContent.
Fast growth disenfranchises longterm members, who find themselves contributing to a very different community to the one they joined. This can cause these most valuable members of the community to go elsewhere.
If the arrival rate of newcomers exceeds the ability of longstanding community members to tutor them, you can run into problems with NoRespectForHistory or ExcessBaggage. The maximum safe growth rate is proportional to the number of "oldbies", and the amount of time each spends welcoming and tutoring, and inversely proportional to the amount of time each newcomer takes to integrate into the community. You can attempt to increase the safe growth rate, but it may be easier to simply limit growth to that rate.
As a community grows, so do its bandwidth costs. Some of these can be reclaimed by donations, adverts, or MicroContribution, but these take time to set up, and if a community grows too fast then costs can grow faster than revenue, just as for any business. In the .com boom, venture capital made up the shortfall, but this is no longer true.
The UserLifeCycle can be modelled as:
This provides you with two good places to limit growth. Firstly, you can stop people from becoming readers in the first place. This can be achieved by using a DeepLinkDefense (especially if the bulk of your growth comes from a specific external source), by becoming a GatedCommunity, by emitting "no-archive" tags to discourage site indexing by google, by reducing the amount of promotion of the site elsewhere, and in other ways.
Secondly, you can stop readers from becoming occasional contributors. MetaFilter requires logins, and simply disables the "new user" page whenever fast growth becomes a problem. You can also make the "new user" process complex and slightly tedious. If your site allows a guest account (perhaps because otherwise LoginsAreEvil), the guest account can be disabled during high growth times. You can also use an EconomicSolution: place the EditThisPage button somewhere obscure, or require it to be turned on in preferences, thus gently discouraging new contributors until they know more about the community.
It is possible to limit growth by preventing occasional contributors from becoming core contributors, or by driving away current contributors. This is counter-productive for the community, as it prevents bonds from forming, and limits grass-roots social solutions. However, because it keeps the community weak and pliable, it is sometimes used by GodKings to avoid the need to DevolvePower.