Well, after the initial form of the European Community was established in 1952, it was nearly six years before any further members joined. The US needed about 8 months to get its first 11 states admitted and then added one per year for the next 4 years, but after that it was 4 more years before a 16th state was added, then 7 years until the next one, and so on.
So rapid growth after founding isn't an automatic thing. Sometimes it happens, sometimes not. Consider how much trouble the Typhon Pact's founding members have had getting along with each other. I think they'd have to sort out those difficulties before they'd begin seriously considering expansion.