I'm not sure what 'entire history of war' you are reading. Did you only read 19th century history or something? If large empires always had an advantage, then y

I'm not sure what 'entire history of war' you are reading. Did you only read 19th century history or something? If large empires always had an advantage, then you would see endless snowballing, and only end up with mega-empire-blobs, and one-world-government. That is not the case. You have period of imperial blobbing, and also period of intense fracturing. Europe fractured after Rome, because smaller polities were much easier to maintain, with fewer layers of bureaucracy to feed. The small Frank kingdom (that only owned half of France) was able to fend off the great Jihad from the Umayyads that had crushed the Visigothic Spain. Whereas if you took out Rome, or the Chang-An of China, the rest of the empire would be paralyzed and unable to act.