Part of it is that many pre-modern armies also had extremely limited ability to estimate the numbers of their enemies, as they often lacked professional scouting units who specialize in such matters - even some early modern 'rationalized' attempts at scouting with collations of multiple scouts' record-keeping and long-distance observation tools like binoculars, like the Pinkertons in the US Civil War, could come up with double or triple the actual numbers of the enemy.
On the other hand, Procopius, a Late Roman/Byzantine writer, accuses Emperor Justinian of killing a trillion people ('a myriad myriad of myriads' - '10,000 * 10,000 * 10,000'), and 50 million in Libya alone. So uh, there's definitely a taste for big numbers with some pre-modern writers.