Why the many mega-pop worlds exist in Deneb (Devlin's Thesis)
Out-of-game
The clearest understanding of the existence of 9 worlds with populations in the 10's of billions (Pop A) in Deneb is simple on two hands: The first was shoddy work and Sunbane generated UWP's; the second was looking ahead for the possibility of these worlds being used as a reverse immigration wave to repopulate the Shattered Imperium (Megatraveller/ post 5th-FW era) with.
In-game
Deneb was settled around the 100's Third Imperial, as Corridor Sector had been settled prior & was named in 140 (MgT-Library Data). It was a beachhead Sector after fighting/ clearing off Vargr interlopers to gain access to the mostly unsettled worlds of the Marches (150 thru 165 contact by IISS to Sworld Worlders & Darrians--MgT Aliens: Darrians). Maps of pre-1st FW in Aliens: Zhodani reveal that by the 570's Deneb was already established as a Sector.
So we may presume Deneb was "settled" between 100-450 TI, and rapidly. The colonization effort to take the marches would take industrial backup, people, and ships to accomplish. At some point, the immigrants from around the rest of the Imperium would 'dry up', so these factory-wrought worlds that spearheaded into Deneb Sector would also have the ability to reproduce their factories, along with workers, and be shipped to likely worlds to exploit in the Marches.
This plan gets interuppted by the Zhodani in the 1st & 2nd FW, and the Imperial Civil War & Barracks Emperors--the lack of clear guidance and the need to slow down and assess the Zhodani halts this grand immigration-factory & worker schematic. But the worlds that remained in Imperial hands needed to get further along & more self sufficient.
Here is where the governments to accomplish this goal mayhaps fell to being led by the Charismatic Dictatorships (ASHASI, ASKIGAAK, LILAD, VINCENNES, & BORLUND); the Technician-Priest Theocracy of DENEB, and the Non-Charismatic Dictatorship of GIIKUSU, the Civil Service Bureau govt of PIKHA, and Impersonal Bureacracy of BEACON seem to be the singular exclsuions to this trend.
Deals made with the Imperial megacorporations assured jobs to these populaion-burgeoning worlds, and Tukera Shipping granted them the means to move their manufactured goods off-world to the Galactic market place, and brought with them the tax revenues to their governments. Yet while the *awaited* day to ship off new colonies of workers complete with their factories dimmed, some of these dictatorships saw a way to maintain and sustain their kickbacks, their license to tax and spend, and keep their populations essentially captive worker-bees, and allowed only E-class downports to be built (ASHASI, ASKIGAAK, BORLUND, & GIIKUSU).
Several other High-Population worlds like ENDUP did similarly, seeing the mass-colonization craze die down in the uneasy 350-year peace that ensued. As long as their goods made it to market and they kept their populations employed and busy, they had more or less a free hand to abuse their human rights and liberties (ENDUP/LUCIFER--RSB), and the Imperial Nobility could wink away the ill deeds of their rulers towards their people.
Granted, several worlds notably PIKHA & VINCENNES had law levels that were less strenuous on liberty and freedom of thought than the others (LL-6), and VINCENNES & DENEB offered careers outside of their worlds with Imperial Navy & Imperial Marine recruitment., but the trend of the others was set.