NotAYakk said:
Districts are limited by the size of the planet. Size is literally size, larger planets are larger on the system view.
They are converting a certain (usable) area of the planet to that purpose, be it factories, forges, urban spacespace, rural farming, mining or power plants.
At least, that appears to be what the rest of the game is telling us.
So, you can have a different belief, but mine is that districts represent actual space used on the planet.
You'll still have some suburban and lower density areas around cities and the like. Converting to a planet city is like going from a planet of new jersey suburban sprawl to a planet of hong kong high rises everywhere.
Ok... let's go by "each district is fixed amount of space". Since each district provides the same amount of jobs, living space and jobs regardless of planet size, they must be of a constant size. This also makes the ecological ruination of one district on a larger planet less impactful to average habitability than on a smaller planet since there is a lot more space to go around. Mathematically it is quite easy by factoring in the total number of districts into the formula instead of just assigning a flat malus per disctrict.
But while we are at it: How is habitability to be factored when we a speaking of artifical environments like habitats and ringworlds? In these system there is no ecosystem to speak of since all harmful things would be filtered out in the atmosphere.
To take it even one step further: Does a subterranean city have the same ecological impact on the ecosystem has a surface city? Does a water turbine powerplant has the same impact as a gas powerplant? What about planets tomb worlds which have a low habitability because the atmosphere itself is more toxic that anything you could throw at it by production?
The basic question also is: what benefit is there in adding a mechanism to a game and therefore consuming system resources if it is just there to have "habitabilty telling a story"?