I've noticed, at least in my last game, that planets from the Arid family of planetary types with dense atmospheres (and thus high fertility) appear to have higher population maximums than similar planets of the same planetary type and size with thin atmospheres, and I can't find anything obvious in planet_types.lua changing the biome sizes for the Arid family with high atmospheric density. What's the change in their biomes for high atmospheric density? As a quick check- it appears that for a medium arid planet with a thin atmosphere, Phidi population maximum is 3.8 population units and Human population maximum is 6.75 population units. With a thick atmosphere, it looks like 4.8 units of Phidi or 7.75 units of Humans.

... can that possibly be right? That looks like it might be a unit of 'bonus' biome as granted by Bionomics or Arcologies. Let me check something, then. Small steppes should have 1 unit of arid biome and 1.5 units of ice biome, so we should be looking at 2.3 units of Phidi maximum on a small steppes or 3.75 units of Human maximum. Compare my small steppe (with dense atmosphere) in this game, and we're looking at... 3.3 Phidi population maximum or 4.75 Human. Yeah, that's a unit of 'bonus' biome all right.

Is this intended behavior? I don't know if I should report this as a bug.

Yes, Arid worlds with a Normal atmosphere have higher max pop than one of the same size with a Thin atmosphere. Does this seem illogical to you?

Not at all. The implementation by adding 'bonus' biome type is what surprised me- I thought that was reserved for technology. I would have expected something more like adding half a unit of Forest biome that scaled to planetary size. I originally was asking what the added biome was, since I didn't see it in the .luas, but sitting down and doing the math answered that question, and in the process raised the question of whether the way it was implemented was intended.

Also, it appears that the unit of bonus biome they get persists after terraforming, which probably shouldn't be the case. Arid worlds with a native biosphere should support larger populations, but this shouldn't make them particularly hospitable once upgraded to garden or island worlds.

Edit: going to post this in the bugs thread.