Indeed, we could find many valid reasons for these structures, but this is not our intention. Rather, it is to know what exists and its function in each case and what depends on the degree and quality of the action – which parts of the garment clothe which parts of the wearer. And we will understand that the particular degree of action requires specific differences in the way one power clothes another. (Differences in the mode of government of the outer garb compared to that of the inner wearer are determined by the extent to which the one clothes the other.)
From the fact that a particular light is great we can infer that it has to be so, because a major action is needed. (Thus Mevo Shearim ibid. explains that the Head of Arich needs greater light than the body, and therefore each of the Sefirot of the Head of Arich clothes an entire Sefirah of Atik.) If the light is small, it is because only a minor action is needed. It is sufficient if we understand this, because the wisdom lies in knowing what is the effect of this and what is the function of that – not the reason why it has to be specifically this way.