Finally, one of the few good scifi isekai appears. I only had to wait five years.
Consider the population of the Mongols descended from the Golden Horde in Western Russia. At the time of their exile and ultimate extermination, they numbered 200,000. Or, consider the Cumans, small enough that their ethnic identity was ultimately exterminated in a single battle gone wrong against the Eastern Roman Empire. Both culture had hundreds of years of rich military history, yet their populations never reached the many millions achieved by their tributary states. All of the settlers from these scifi planets probably had the same cultural background, so they probably hit a comfy population level. Most human cultures did not wildly breed. For example, British Java had scattered indigenes, including literal bronze age nobodies out of an isekai who got vaporized when that supermassive volcano erupted. Over time, the hybrid Chinese-indigine-British agriculture dominated and Java came to see a baby boom a full century before the rest of us. Every culture was quickly subsumed, but previously, even with migration and war and exploding volcanos, Java retained a rich variety of cultures mostly alien and incomprehensible to their fellow Javanese. I presume the default state off humanity is not to exploit the land till it dies, because such cultures die, selecting only for those culture that can strike a balance... and, irrespective of tech level, such cultures only reproduce very slowly and might hardly grow at all. Since every scifi planet is a mini-Java without external factors influencing development, it wouldn't be surprising that low-fertility space Amish might be selected for by an Empire for colonization efforts. After all, your reach is vast but not limitless, and what better way to control the stars than to seed them with vibrant but mostly compact and easily controllable cultures that have little chance of getting uppity and starting Empire on the own?
I had a while blurb on technology that got deleted... Basically, a modern welder or electrician is stupider than a hunter-gatherer because
1. Less adaptable
2. Professional skills inferior to ethnibitany etc
3. Dependant on civilization superstructure
4. Probably less well-read/cultured (most people don't even read a book a year; reading isekai trash puts you in the 90th percentile of well-read humans) with inferior mental landscapes to those of tribals unaccustomed to flashy led screens doing the visual thinking for them.
Basically, if the galactic empire had an explicit policy of inhibiting liberalizing interstellar trade, a 200,000 person colony staying small is probably reasonable. Most cultures, NOT MOST PEOPLE, do just fine left to their own devices and die out only when they for example get the strange idea of building giant easter island statues, and they typically slowly improve their standard of living anyway. Of course, this particular isekai planet better have a pop in the tens of millions because the first thing genghis khan told his people is BREED because he needed moar dakka.