For Choice of Rebels, the answer is definitely the former. It began as a one-game concept (as the text in death.txt suggests, for those who have a copy and can read the code), had grown to two games by the time I officially pitched it to CoG, and continued to grow in my head until it reached five games–where I intend to plant my flag and go no further.
The decision had nothing to do with making more money; I’ve no idea whether five linked games will ultimately make more than five separate ones. But–unlike @jeantown, who’s stuck to her guns on the idea of not publishing the completed Guenevere Game I until she’s finished the whole series–I wanted to start publishing when I reached the right milestone, and the end of your Robin Hood bandit-in-the-woods phase seemed good.
But are there really that many CoGs with sequels? I feel like the majority are still standalones.