What is the in universe reason for pop growth slowing down as more pops are in the empire?

[The Founder]

Apparently, you cannot have more than 1 POP assembly type on a planet. So, when my masters of evolution acquired the ability to build Clone Vats, I realized I already had robot assembly plants on all my planets.

So... which should I use? I'm locked into evolution mastery at this point, so I can't just decide to become robots.

Go cloning Vats. Replacing Robot with organic Assembly is their actuall purpose.
You propably have to phase out Robots anyway - the sooner you start the more organics you will have when you do unlock Genetic Engineering.

If that was true then empire-wide carrying capacity would be a thing, which you could increase by settling more planets.

We still need a mechanic to affect pop assembly. While bodged as heck, the "increase cost/pop based on number of pops in empire" is a solid way to do that.
The increasing cost nicely simulates having to counter losses.
And to keep the Robots from becomming another out of scope grower again, they have to share the limit with organic pops.

Population was quite stagnant before the industrial revolution. Developing countries are just having their industrial-age growth spurt later, a growth spurt which ends by stagnating at a new effective carrying capacity like the West has. Also Russia has lower birthrates than the USA and it has an awful economy.

The industrial revolution increased the carrying capacity (humanity finally started building some city districts) while also allowing one generation of "replacement children" to reach procreation age.

 

[CountAccountant]

Relativity. Pops continue to reproduce at the same rate they always have in their own frame of reference. But as stellaris pop mass increases, the resulting stellaris-pop-gravity bends space-time to the point that an outside observer perceives 10 years passing before the population of their ring world increases from 3 to 4.

This also explains what happens to the galaxy itself from the perspective of an outside observer if enough pops are allowed to grow by late game without being purged.

 

[Kayden_II]

It's mind-boogaloo-time ( "universe reason" ) again in order to "justify" this ... stuff ( "pop growth slowing down as more pops are in the empire" ), right ?

 

[Objulen]

Increased (non-specified resource) overhead for a growing empire and demographic transition are probably some of the better in-game explanations.

While demographic transition wouldn't usually apply to gestalt empires, they could still be affected by the increased overhead for attending to a larger populace. Hive minds need to spend more attention dealing with infrastructural needs, and machine intelligences could start facing performance issues, needing to spend more assembly resources maintaining their population.

That would be better represented by a scaling upkeep cost, not pop growth slow down.

Reducing pop size by implementing pop tiers, directly or indirectly, would be a better solution IMHO.

 

[Gaspi2]

I Don't see the logic of "boom in rich countries slow down". How I see it that games emulates now this:

we have new colony on Mars (1000 people), but there it takes 90 months for ladies to deliver a baby because there are already 10 billion people on Earth.

This rich and developed country and plateauing pop growth is already simulated by the S curve of every single planet individualy. Why would colony that is x million light years away from different colony give a damn about that there are already 10 billion people stuck on planet like sardines and make babies at slower rate?

regarding the maintanance thingy that's what empire sprawl is for. That's what maintanance drones/amenities is for. Every species on Earth has one obective - to safeguard their genetic code and as long there is place and good conditions to live they will multiply at the maximum rate. The population of deer in one location won't be stagnating because there are plenty of deer in some other location.