The size of people took a nosedive in the transition from gathering-hunting to agriculture because of diet. (Gathering was far, far more important and provided most of the food for most “hunter-gatherers”; the backwards name reflects the macho bias of the first anthropologists, who talked almost exclusively with men and didn’t notice the importance of women in either their own societies or those they were “describing”).
Gatherer-hunters typically worked about 20 hours a week and spent the rest of the time playing, doing art, singing, and drinking beer (almost a cultural universal). Ag dramatically boosted the time spent working, while it drastically increased its monotony. That trend has mostly continued, with a few temporary reversals and attempted reversals. (Luddites, labor rebellions of the 1880s, etc.)
The richest 7-10% emit half the greenhouse gases; the poorest 6 billion emit about 20% of the GHGs. Our problems now–ecological, (especially climate) war and others–are overwhelmingly caused not by our numbers but by the richest few percent. And inequality and the insecurity, lack of education etc caused by inequality are now the main causes of population growth, which is in any case almost certain to level off and decline by 2050 in the face of exponentially rising death rates because of climate catastrophe. We certainly should do the things that shown to slow population growth because they’re good things anyway–equality, education and empowerment for all especially women, security in hard times, sickness and age. But it will make virtually no difference in the current crisis, which we have to solve in less than a decade or face unimaginable horror on a global scale. And then, either way, it will be moot. We need to reduce inequality and the harm the richest few percent do, aiming toward fair and equal distribution globally, while switching to clean safe renewable energy, reforestation, small-scale low-meat organic permaculture and benign biomimicing craft-industry everywhere.