As it turns out, the reduced number of ARSes on smaller maps was something I had intentionally added a while back, so it wasn't related to the home planets placement logic at all. I've decided to reverse that prior decision for sure, though, as I think it makes the smaller maps too hard. From upcoming O's release notes (O will be out tonight):
-In recent releases (since 1.201 or so, most likely), on maps smaller than 80 planets there were often too few Advanced Research Stations. This was initially by design, but it made the smaller maps harder than expected. There are now always 5 ARSes on every map, and existing savegames will be updated on their next load to include the correct number of ARSes.
-On very tiny maps, especially 10 planet maps that have 8 planets starting out belonging to players, this can lead to duplicate ARSes on individual AI planets.
-The population algorithm for ARSes is now better -- before it was only ever seeding them on planets belonging to the first AI, but now it will seed them on the planets of both AIs. This will affect the layout of existing map seeds, but not existing savegames.
-The initial population algorithm for Advanced Factories is now better -- this really only affects very tiny maps with large numbers of starting planets. All AFs are placed in enemy territory, but this could now include home planets of the AIs, and multiple AFs per planet if needed. The number of AFs will normally be 2x the number of starting player home planets, but it will also now never exceed 2x the number of starting AI home planets. The changes to this algorithm will not affect existing savegames.