You can read the summary of Kohr's thinking at Zero Hedge.
It isn't just nations. Yes, there are economies of scale, but there is also a point of diminishing returns. Up to a point, it makes sense for farmers to "go big" as the odious Earl Butz insisted back in the '70s, and it makes sense for corporations to grow. Everything, though, from a human body to a business to a nation reaches a happy, healthy, natural maximum. Beyond that point, growth can become destructive, like cancer.
Kohr posits the Power Theory of Aggression in which he suggests that there is a critical mass of population that becomes dangerous:
But there is one element capable of accumulating its physical substance so far and so unequivocally beyond the critical limit that no force on earth can check it. This is the immense collective bulk of the most courted organism of our time, the human mass, the people which, at a given size and density, not only generates the ideal condition of anonymity at which a greater number of individuals can, without danger of detection, sweep up critical quantity of power than would be possible at the more translucent lesser densities; at a given point the mass becomes itself so spontaneously vile that, in addition to the increased quantum of individual misdeeds, performed under the cloak of its darkening multitudes, it begins to produce a quantum of its own, and wholly detached, badness that bears a relationship to its size, but not to the nature of the human molecules composing it.Kohr liked the American model of states distinct from the federal government. It was still a model that worked fairly well even going into the late '50s when he was writing his treatise. But population-wise, we were roughly half the size we are now. Agriculture was still mostly family farms, most men worked in productive fields like mining, manufacturing and transport. Government, especially the federal government, though already too intrusive for some, was still limited. Public sector unions were not the force they are today, and schools were still mostly under local control.
When this social volume is reached, everything becomes predictable, and nothing preventable. The question is then no longer: how many crimes will be committed, but who will choose in the freedom of his will to be the criminal tool of the law of averages
More unified, global governance was supposed to bring peace, prosperity, and security. How is that working out for everybody? I am sorry that my Scottish kinfolk did not opt for independence because I tend to believe, with Leopold Kohr, that the preservation of liberty and the answer to our problems lies with secession, decentralization, and dispersion of power.