Simple ones probably do. Because a small country has both less buyers and less sellers. Consider the people of the world to be a graph. Each person is a vertex

Simple ones probably do. Because a small country has both less buyers and less sellers. Consider the people of the world to be a graph. Each person is a vertex, between each pair of people is a directed edge indicating how much they bought/sold from the other person. Assume every edge is selected independently at random. Consider a country to be a set of people. The average weight of the edges crossing in/out of the set (treating incoming edges as positive and outgoing edges as negative) will be zero no matter the size of the set. --- The bizarre part here is that they went from "models often assume this" to "it's something we should attempt to force to be true via taxes". Trying to force the world to fit your known to be incorrect model assumptions... doesn't make any sense as a policy.