Interesting idea to think about. If we want to go full realistic, one would have to account for a high-enough birth rate to sustain population replacement rates if they killed half of their children (lore states that there's a 50% chance a goblin child will be Halfling and vis versa, so for such a brutal population, 1/2 of the children would be killed). This article https://www.thoughtco.com/total-fertility-rate-1435463 says that in developing countries today, people need roughly 2.3 children per couple in order to sustain population replacement. I'd bump that higher, to a friendly 2.5 for a Goblin community in a harsh environment. If they're killing 1/2 their children, every mother would need to be giving birth to a litter of, on average 5 children.Also consider that originally the bitter reach book mentioned the harsher crueler more survivalist community of goblins with no mention of halflings at all. Halfling children in that case could be left out to die as being too weak. It's not what they ended up settling on for the book but it's a thought. A goblin or halfling community could just have a lower birth rate because some children "die in child birth".