The Battle of Cannae supposedly had 86000 Romans face off against Hannibal's 50000 men. A battle involving that many people in modern times is unthinkable with the exception of The Battle of the Somme which had mechanization. How accurate are the recorded sizes of ancient armies?
This older answer might be of interest, from /u/flavivsaetivs, as well as this thread with discussion from a number of users, although I would note that <100,000 soldiers per side is quite small compared to modern campaigns.