Why did classical battles did tend to be larger than medieval ages? One particular example I am thinking of was the battle of Stamford bridge, which had around 30,000 participants, compared to the battle of Wattling Street which had supposedly had 10,000 Romans face against 230,000 Britons. The number of Britons is of course likely exaggerated, but even if the Romans were outnumbered 1:3, that would already make for a larger battle than Stamford Bridge.
This is a complex matter (some authors like Delbruck thought that the classical numbers are very inflated) but one may point out to logistics - classical states were much better able to extract and stockpile resources (human and material) than high medieval polities with their fragmented political authority and erratic currency.
As for the Romans' barbarian opponents, there we often have whole tribes on the move, which account in a different way for the relatively large numbers, whereas in medieval battle we do not encounter such population movements.