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I think I expressed this unclearly. Lifespan is more tightly correlated to brain size than body size (brain size scales disproportionately with body size). The author gives the example that if lifespan was correlated with our body size, humans would live 20-25 years, but since lifespan is correlated to our brain size, our expected lifespan is longer.
I agree that the correlation doesn't necessarily point to a causal linkage.
Related to the quantity argument, the book talks about how there was a green revolution of sorts in the cocaine business. Between 2005 and 2011, the amount of cocaine that could be extracted from coca plants increased by 60-100%. In that time period, the quantity of coca plants decreased without the quantity of cocaine decreasing.
I have another data point that supports your modern day transportation constraint of 10-20%: skyscrapers. Engineers can build skyscrapers that are more than a mile high, much taller than our current skyscrapers. However, the taller a skyscraper gets, the more of its footprint is devoted to elevators and and associated equipment. At a certain point, your skyscraper just becomes a useless bundle of elevators. Right now, developers stop adding floors to a skyscraper when elevators are 30-40% of its footprint and 2-10% of its energy costs. The true transportation costs are probably slightly higher because they also include the costs of elevator installation and maintenance, and are probably roughly in line with your transportation constraint.
I agree with this. A lot of the things I do remember are things I regularly use for my job.