Ooooooh, boy, the Classical Vs Medieval Army Sizes question. Just about everyone I've ever read or talked to about this matter agrees that it's A Thing, and that your typical Medieval European army is going to be smaller than the armies raised by the Greeks or the Romans...but as far as I can make out, we still have no clear answer on why this is the case. (My prevailing theory is that, for the specific case of the Romans, a Roman is actually three small Latins stacked on top of each other, and the given army sizes for Roman armies are actually counting individual Latins. Thus a Roman army size should be divided by three to get a picture of what their confused opponents saw from their side of the field.)
That aside, some posters have attempted to grapple with the question, so while we wait for someone to take on this problem, here's a few previous posts for your perusal, oldest at the top: