1. Population.
2. Increased raw production from economic starbases and interstellar governance techs.
3. %bonuses from planetary buildings and other starbase modules.
Increase pop by 10%, increase raw production by 10%, and increase planetary bonuses by 25% (a factory) and you get 1.1*1.1*1.25 = 1.5 = 50% increase in production.
The 50% increase in production means that you can get the NEXT 50% increase in production even faster. Because you've got more production, you can spam out factories and constructor ships EVEN FASTER THAN BEFORE . AHAHAHAHAHAHAHAHA!
Once you get to this level, you can now spam out constructor ships and factories SUPER DUPER FASTER compared to before, which will increase your rate of factory and starbase spam even more (game-time wise).
Once you've reached the SUPER DUPER LEVEL OF PRODUCTION, you can spam out CONSTRUCTOR SHIPS AND FACTORIES EVEN MORE SUPER SUPER DUPER FASTER than before, which will allow you to build factories and starbases EVEN FASTER.
Once you've reached that new exalted level, this allows the SUPER EXTRACALIBER DUPER DUPER FASTNESS of constructor and constructor/starbase spam until pretty much all of those things can be built in 1 turn.
At which point production acceleration levels off because you've filled all of your space with economic starbases and planetary tiles with factories and what not.
Overall, the game is effectively an economic rat race to reach SUPER EXTRA CALIBER DUPER DUPER FASTNESS WITH AS MANY PLANETS AS YOU CAN GET. The first one who can reach SUPER DUPER EXTRA CALIBER DUPER DUPER levels of economic production wins.