Typical decisive battles in Roman times involve 4+ legions - around 20,000 men + auxiliaries giving a total of about 30,000 men, on the Roman side only. Battles in even the late middle ages and early modern age typically involve 30,000 men in total. Even Alexander the Great mustered over 40,000 men in tiny Greece with an estimated population of 3,000,000 whereas mighty France with an estimated population of 13,000,000 in the years before the black plague could only barely match that number at Crecy.
Given the increase in population over 1,500 years later, why did the early modern world not see more battles of ancient classical scale?