From my (very limited) research I see the largest battle of the medieval period was around 30,000 men per side (Grunwald) where it seems that Rome could, many times throughout its' history, field 60,000-100,000 troops for a single battle and quite often come up against enemies with the similar or greater numbers. Did the militaries of the Roman era understand something unique about recruitment and management of large armies or was there outside factors or just flat out less people?
There are multiple factors that play into it. More can be said on the disparity between Classical versus Medieval army sizes, so if anyone would like to type up a post of their own on why this was the case, please don't let this post stop you! Insight is always, always welcome.
For the meantime, OP, the following previous posts may be of interest, oldest at top: