At one time, every large human group was a small human group. What are factors that could have made it grow faster than other groups?
While in a minority in a region, it could encourage isolation from competing ideas. Within families, the group could discourage contraception and encourage large numbers. It could encourage conversion into the group.
It could discourage dissent. Alternative groups could be suppressed by removing competing ideas or by removing competing individuals.
But total numbers in the group will not rise if people leave at the same rate that they are joining. So it could be made difficult to leave the group. Once any group has taken control of land and business, there will inevitably be social pressure on others to conform. Or penalties for leaving the group could be made even more explicit.
Many of these factors could apply to any group that is growing. But does the fact that something is growing mean that we should encourage it?