I went on a delve in the sub on just this subject a few days ago, but considering what I managed to turn up, I think this is ground that could use some more treading.

u/FlavivsAetivs examines this same matter in this thread, with further discussion beneath.

u/J-Force looks at the Roman army of the 6th Century (ie, the army under Justinian and Belisarius) in this thread. Not quite a direct answer to your question, but he does go into reasons why Justinian's armies are much smaller than is typical. The Eastern Roman/Byzantine Empire is an exception to the Medieval trend of smaller armies than Classical Rome.