I'm not exactly sure, but I think the author is asserting that planets are likely not to be much larger than ours [1] and if that is the case, larger aliens means they require more space and that puts an upper bound on population size (e.g. the example he gives is if there were as many humans as there were ants, then together all humans would require more energy than what hits earth from the sun).
Wouldn’t it be the inverse? A larger planet would have smaller creatures due to higher gravity, thus the need to expend more energy for a similar relative size?
Why is that? If aliens are 10X larger than humans, but their planet is also 10X larger in surface area and resources, why can't the population be similarly large?
Also, isn't it similarly likely for some aliens to be on average smarter than us, due to some freak of evolution, and some to be dumber..
Genuinely curios if I'm missing something.