Exponential growth, and the power of Benevolence Level III and super traits Patriotism and Colonizer.
Also, there's no maintenance costs to speak of, apparently.
I look at the people who actually try to build up their world early game and I'm like: TROLLLOLOLOLOLOLOLOL.
Just build colony ships with different engines (slower and cheaper colony ships for planets close by. Faster and more expensive colony ships for the distant plants). Literally that's all you have to do to win the game. You don't even need to put any population into the colony ship. Just colonize planets as aggressively as you can and win.
Once I get Benevolence Level V (research line) and the Thalan Hyperion Matrix, ALL of my colonies are generating 7-8 science per turn. With 100+ colonies, that's close to ONE THOUSAND research points per turn with my research slider at ZERO.
Problem is: game is too easy.