It always bugs me a bit to load several billions onto colony and troop transport ships (yes I know I can decide to send less, but the game is geared towards large numbers, and esp. for invasions you need them)
Fine, we may easily say one colony ship represents not only a single vessel (or does it?), but a number of them colonizing the target planet in one larger wave.
Still, if your homeworld has say 5-10 billion people, does it really make sense to assume several billion of those are leaving in one go? Wouldn't there rather be a way smaller first attempt, then over time a more or less steady influx of colonists up to a certain level?
At the same time, for ground invasions you send massive parts of your population away as soldiers, and on the defending side everyone from the newborn to the oldest grandpa seems to join the military to fight back.
IMO it would be kinda cool to have an overall smaller dedicated military force for ground invasions, that is not constantly "spent" in every invasion, but could get better with experience, buildings and technology or even ideology. Losses would still have to be replaced, but not by swapping billions of people around constantly.
The same type of military force would be your opponent on the defending side, so you have to defeat the enemy military, but not every single guy of another species living on that particular planet. You could even get mixed populations after an invasion, though that would add another layer....
Of course, this could easily overcomplicate everything. However, maybe some of this could be developed into neat features...