In VB6 aurora, I always colonized Luna and Mars, and one would get my best ship construction administrator, and the other would get my best ground force training speed administrator. When you are talking +50 or even +60 ship building, that is an enormous savings, especially as it applies to retooling and shipyard expansion as well.
Lower population worlds having a smaller service worker population. If you have a 25 million pop world, you can employ most of them in construction or research. If you have a billion pop world, 750 million are going to be in service jobs.