daryen,
Well, I think your right, in the long run.
However, in Markasher, what we have is a world that remains TL-6 after 1100 years in the center of the Imperium. I'm going to guess the culture is pretty hard-core anti-change. 1100 years is a long time for the Imperium or the Megacorporations to run scores of society modifying operations. The Imperium will be trying to make it open up for contact and the general profit of the Imperium. The Megacorporations will be tyring to open up the market for their profit. But everyone will want a market of four billion people open to general traffic.
And yet, again, it's still TL-6.
I think the local populace and the history of its admittance into the Imperium is going to have considerable influence on its development, especially if the world as conquered.
Also, since TL is one of the main points that determines the size of an economic system on a world, and therefore, it's taxation, the world may be deliberately holding down its technology, and therefore its taxibility, based on past history and a desire to stick it to the Imperium for conquering it a millenium in the past. It could be a part of the national heritage.
As we can see in various parts of the world today, people are willing to carry a grudge for a long time.
Or perhaps something went especially wrong here in the big psycho-historical experiments a few centuries earlier? Who knows? (I may save that explanation for another world.)
In general, though, I'll keep the starport size increase more firmly in mind for any other High-Pop vacuum rockball worlds that I work on.