Most military campaigns involved a HUGE percentage of Support personnel.  The actual fighting force, the "tip of the spear", amounts to only about 10% of the total manpower.  Similarly, on a planetary defense, though technically the entire population is enrolled in the National Guard, planetary militia, Volks Grenadiers, whatever, the actual combatants are a much smaller number.  And on the defense, most won't even see combat because most are deployed in defensive positions that never see combat.  ["The Attacker gets to choose where to apply the bulk of his military might.  The Defender, on the other hand, must try to defend everywhere."  It's why offensive geniuses like Napoleon and Frederick the Great so often defeated much larger nations and armies.]

You realize you just explained how things worked in MoO2, more or less? 

Quoting aerowreck,

reply 3

You realize you just explained how things worked in MoO2, more or less? 

I never played any MOO game (shame on me )

Despite the suggestions of the population growth branch of the tech tree, the game's population growth behaves nothing at all like biological population growth. The expected growth per time period of an animal population is

P*(b*H(Z) - d) + I(E)

where P is the current local population, b is an expected birth rate per some number of people per time period, H(Z) is a limiting function which modifies the population growth based upon the resource availability vector Z (which includes food availability and population pressure), d is an expected death rate per some number of people per time period, and I(E) is an immigration term dependent upon the vector E which includes factors such as total empire (or arguably galaxy) population, immigration and internal migration controls in the empire, immigration controls at the planet, desirability of the planet, pressures pushing people to depart other worlds, et cetera.

Instead, we have a population growth which is a constant 0.1 per turn, plus some modifiers. The names of the growth-boosting things may suggest that population growth is a result of biological functions, but the actual behavior of the numbers suggests otherwise.

Population growth, even for factions which supposedly have populations which grow by sexual reproduction, does not behave like a biological process. It does behave similarly to an enrollment (or recruitment) program which is provided with a constant amount of resources over the course of the game, with some modification for the attractiveness of the location (i.e. quality of the local medical and recreational facilities, etc) and local popularity of the government. It's not even like it's difficult or time-consuming to use something similar to a natural population growth model if you wanted population growth to reflect birth rates; the very basic model P(T+1) = P(T) * (b + 1) adds at most one multiplication to the current model of P(T+1) = P(T) + c, which should make rather little difference in computation time.

If you accept, as you claim you have, that the population growth is not in any sensible manner a function of biological reproduction, then to use that population growth function as the justification for the claim that a planet full of Yor becomes a planet full of humans upon conquest simply because the population growth goes from the on-demand set growth of the Yor populations to the population growth function used for biological populations is nonsensical.

If my empire's original species is not synthetic, then any colony of mine will have a population growth is 0.1 per turn with some modifiers, regardless of initial population at the colony, regardless of empire-wide population, and regardless of the population type which was presumably present upon the planet when I claimed it. In 50 turns of time (just under a year), I will have gained 5 additional population whether I started with 0.5 population or 5 population or 500 population. It makes no sense whatsoever to use a population growth rate which is so disconnected from the actual population present to infer information about the type of population which is present. The game mechanics are such that if you have a planet with a population of 0 and your empire is not set to be synthetic, then that planet will grow at a rate of 0.1 population per turn before modifiers (none of which include, for example, a -100% growth because there aren't any people here capable of reproducing). In what way does this differ from a planet with 5 synthetic population and 0 biological population growing at a rate of 0.1 population per turn before modifiers?