At the moment the population base growth is a fixed 0.1 per turn. This means independent of the current population size or availability of food surplus the population will grow by 0.1 (plus any augmentation by growth modifiers).
You can accelerate your growth by building certain buildings (hospital, etc) and/or researching growth modifying techs. But major growth can be achieved by producing "Birthing Subsidies" on strong production planets.
For argument's sake i will ignore the modifiers (including happiness).
If you only look at the "natural" growth (i.e. not producing "Birthing Subsidies") it is rather strange that a planet with a population of near zero will grow as fast as a planet of 100 billion people. The effective birth rate is in the first case 100% (0.1/0.1) while it is in the second case 0.1% (0.1 / 100).
A far more logical behaviour would be: The population size should be the driving factor for population growth and the availability of food should be the limiting factor for the growth. If there is sufficient food surplus the population growth should be a percentage of the current population size. If the population approaches the food limit the growth should slow down.
Example (for population smaller than food production) with growth percentage of 2.5%:
PopulationNaturalGrowth = 0.025 * CurrentPopulation
GrowthLimiter = 1 - (CurrentPopulation^2 / MaxFood^2)
NewPopulation = CurrentPopulation + (PopulationNaturalGrowth * GrowthLimiter)
This will give a more realistic growth curve then the fixed growth of 0.1. Underpopulated planets (0 ... 2,5) will grow slower; regular planets (2.5 ... 16) will grow in roughly the same rate as the current implementation; lush planets (16 and greater food) will grow faster, the greater the food surplus the faster the population will grow.
Of course the modifiers should still have a major influence on the actual growth.
The problem with constant growth
The constant growth means that you are not penalised for depleted planets or rewarded for having a food surplus. Any negative influence of population pressure on approval can be compensated by other means. This leads to the valid strategy of "near zero population" colonizing.
A method of fast colonizing the free planets is by using empty cheap colonizers. You can achieve this by (for the Terrans):
1) unloading your initial colonizer on Earth;
2) send it with minimal population to Mars;
3) make Mars the primary sponsor of the shipyard;
4) make Earth the secondary sponsor;
5) start building cheap colonizers. This will deplete Mars and the colonizers will have near zero population, but that does not matter. What matters is that Earth will have maximized production;
6) colonize any planet you can lay your hands on;
7) repeat until there are no more free planets;
8) keep transporting population from a number of small planets to your main production planets until they are stuffed to the gills;
Depending on the distance to the first/nearest habitable planet the break even point (without any changes to the production distribution wheel and leaving social vs military production at 50%) is reached around turn 31. With production tuning it can be even faster.
The problem with proportional growth
Well managed large planets will reach their population limit a lot faster. This means more production, more wealth, more research, net effect: a lot shorter game. So the max growth per turn should be capped.
While i am on the topic, a nice curve for starvation might be:
MaxDeathRate = 0.75
OverPopulation = CurrentPopulation - MaxFood
StarvationRate = (CurrentPopulation / MaxFood) - 1
NewPopulation = CurrentPopulation - min (OverPopulation * StarvationRate, CurrentPopulation * MaxDeathRate)
Guys (and Girls): Can i get a Yea or Nay ?