I obviously know what per capita means. My point is interrogating the domains where that metric is useful and isn't. Are we delivering 10x more letters per cap

I obviously know what per capita means. My point is interrogating the domains where that metric is useful and isn't. Are we delivering 10x more letters per capita? Does US mail sent use scale with GDP per capita? In many areas I would expect a 1:1 relation with population, but not GDP. For example, I would expect primary education cost to scale at with population (eg teachers and schools per student). If cost per student has gone up 10x instead, why is that?