Well, that was the easiest way to represent ground combat without getting into too much micro-management. It sucks, but I don't think it is really that bad.

I would have prefered that planets had two kinds of population, civilian and military, with the military being determined by recruiting and research level in a "ground forces tech". Then military transports would only carry military population, while colonizers would only carry civilians. To invade other planets you would obviously need military population, but to defend, since some planets will probably not have any, then the game could generate a "militia". Lets say, 10% of the civilian population would become militia in the case of a enemy invasion. Once the local defenders get defeated, the local civilian population would become part of the invading empire. To simulate resistance, the occupiying empire would not receive income from that planet for, lets say, 10 turns? After that, depending on a "alien ocupation" tech, the ocupying empire would start to receive an partial income, progesively increasing, for x number of turns until the local got asimilated or pacified. The presence of invading ground forces policing the recently ocuppied planet should also reduce local resistance.

Its a fairly simple system, but I doubt it could work with the current building of GC2.

You would need two types of population, military and civilian, plus the civilian population would have to be racially identified, so for example, when the Drengin occupy one of my nice terran planets, the planet window would say: Population 5.2 B Terran, or something along those lines.

Having "mixed" planets might be the MoO thing that I miss most in GC2. I really hope to see an implementation for GC3, along with an analog of the MoO population transport system (you could deliberately change the dominant species on a world by moving specific pop units on or off).

The population displayed only counts taxpaying citizens of your empire.  There are many more people living in your empire who have not been granted citizenship.  Citizens are drafted into military service when necessary.

If it helps.... Think of the numbers being the soldier #. the reason the smaller side can win is 1- they all landed relatively close while the other had to be spread out over the entire world, and 2- look at the milatary score and tech.

I thought the reason the invaders get such a big advantage was air superiority - you've already destroyed their spacecraft so it's assumed that your invasion force can bomb and air strafe with little chance of reprisal. Not to get too political about it, but this a major reason why the USA has been so dominant in most military conflicts over the past 50 years; air superiority.

It does seem more realisatic to think of the in-game invasion numbers as thousands rather than millions, but unfortunately that's incongruent with the tranpost taking X million people out of your population when it lauches. I think the reason behind that game machanic is to limit the number of transports you can effectively use by something other than just your production capacity. Otherwise it would be too easy for high-production civs to spam them. But it is rather simplistic and uh, genocidal, to have it so a) every man, woman and child in your empire can become a deadly soldier as soon as you make a transport for them, and every planetary battle goes down to the last man.