If you play a Prolific species, the colony ship you start has a full 5 billion people on it - which will place 10 billion on the destination. Most planets can only hold 5 when colonised. Suggest that the colony ship granted by this ideology only have 2.5 -3 billion in it (new colony modules have a 3 billion cap). As it is, I added the 5 billion to my home planet while I waited for anew planet to colonize.
This is also an issue with the 3rd idea (Prolific) in this tree = planets have a pop cap of 5 now, so adding 5 pop to every new colony will push it over the maximum pop every time.