I feel like this correlates with nations getting bigger over time and the square-cube law (or line-square law for national borders?) but I am not smart enough at military stuff to figure it out
I've read that it's kind of the converse - as military technology advances the size of a "minimum viable nation" increases. E.g. as gunpowder technology developed, anywhere that couldn't afford to field a gunpowder military got absorbed into somewhere that could.