I set my colony ships at the start of a game to only have 0.5 population on them, the minimum. I noticed that some of my colony's get to 3 pop very very quickly, found this strange so wanted to see what was going on.
I colonize a planet with a colony ship that has 0.5 pop:-
The planet has 0.5 pop and you can see it is only gaining 0.3 pop a turn:-
I purchased a colony ship. The ship is ready the next turn, so the colony should only have 0.8 population. I launch the colony ship(the slider shows 3 population):-
Planet now has 3 population:-
