The answer to this question can be found at the end of Chapter 14. It starts on p. 289 in the paperback edition of the book. Diamond says that this amalgamation never happens voluntarily. Instead, it happens through coercion.
The first way in which small societies amalgamate into bigger ones is through the “threat of external force.” In other words, a small society recognizes that it is in danger of being swallowed up by a bigger society. In order to prevent this from happening, the smaller society joins together with other small societies in hopes of being able to fend off the larger society. Diamond says that this happened with Native American tribes that amalgamated to try to resist white incursions.
The other way in which small societies amalgamate is through actual conquest. Diamond describes this on p. 290. He says that a small society can become strong and can go out and conquer other small societies. He gives the Zulu state in southern Africa as an example of this process.
Thus, small societies only amalgamate into larger ones when they are forced to do so.