Funny, I seem to recall living in a country that interpreted the Enlightenment to mean that the most reasonable government was one that respected man's natural rights, and that in order to protect this, devolution of powers was necessary.
That's a particularly English concept - France, even before Louis XIV, still had a fairly centralized state structure with the Church hierarchy included as one of the estates. Feudalism was fairly centralized within the various fiefs, in many senses - everyone had a superior, and if your superior called you up, you went. The Holy Roman Empire, on the other hand, vacillated in control not because of any theological concept, but because of the nature of German geography making it nearly impossible to control a large area directly, and the Church desiring political control over an area that's across a freaking mountain range. Italy only shifted based on who had the most money at the time - either the merchant families or the Church. It wasn't really based so much on theology - the Ghibelline and Guelf conflicts were about the pope as a political leader, rather than a spiritual leader (which no one questioned).
I don't think you'll find many who would say that the Hundred Years' War was a small scale event - stretched out over many skirmishes, yes, but huge chunks of land were shifted around in the conflict.
It also bears mentioning that Spain was particularly bloody for 7o0 years, less so when the Muslims were in control.
Finally, I question whether it had to do with theology and political philosophy that things were as small scale in the Middle Ages as much as it was technological and natural effects. In regard to the latter, two factors primarily weigh heavily. One is the Black Death of course; it's rather hard to form mass armies when you have to fear a third of your force being annihilated within months from disease. The second is the Medieval Warm Period - because there was much more arable land for much longer during the year than in the Roman period, feudalism made much more sense economically than the previous model, which combined slavery with seasonal workers. Not only that, but it made the cities even more fetid and horrifying, which deterred the population density necessary for centralized control in the modern sense.
Technology-wise, it was still the Dark Ages. There was very little in the way of disseminating information, and even then, the info would be suspect at best. Memeplexes were almost impossible to spread outside the centralized Church hierarchy. The printing press cleared that mess up.
I would say not so much because of Enlightenment rationalism, but more because of the technological developments that occurred at the same time. Controlling armies became much easier when 5,000 copies of the same orders could be distributed fairly easily. Not only that, but the colonization of the Americas basically taught Europe how to do supply chains. The end of the Black Plague and increasing urbanization stemming from population density also helped the dissemination of information that's vital to mass armies.
While Clausewitz is well-known for that statement, it belies an understanding of politics that perhaps isn't as discrete as it could be. Politics answers the simple question of "who gets what", or resource distribution. In the medieval period on the continent, the King got taxes. Why? Because he had the biggest army. The Church gets tithes. Why? Because if you didn't, you were a heretic and executed. Vassals get portions of crops. Why? Because if you didn't, they had strong men who would either throw you off the property or kill you outright. It was all the politics of force. War isn't the continuation of politics - war is politics (this group gets that because they killed the people in their way).
Two final points: one, it's unwise to forget that there existed mass armies of Napoleonic scale in China and India much prior to Bonaparte's era, and without any sense of the European Enlightenment. Two, Sherman's war philosophy was much more existentialist than rationalist - he came up with his "hard war" concept not by asking what worked best, but what war was, and based on that, how it was best prosecuted. Plenty of other generals at the time were trained in rationalist (Napoleonic) thought, but Sherman was the one who made the jump from how war was conducted to what war actually was. Napoleon may have said that an army marches on its stomach, but Sherman actually understood the implications.