One thing that has always puzzled me is the vast difference in the estimation of the size of armies, done by contemporary historians and the ones done by scholars of ancient times.
For instance where ancient scholars put the numbers in hundreds of thousands, present day historians put them in thousands. I was reading about the Battle of Dorylaeum, and the difference in estimates is very large.

So my question is how exactly did they count the number of men back in those days (or in days of antiquity), and how do historians today estimate those numbers? And why do they always reach on such different figures?

I mean I understand that people back then might have exaggerated the numbers to make things seem more...impressive...but still how do the people today know?