I don't see how Roman armies could have been 400,000 men large. It just wasn't possible to supply that amount of men in one spot back then. Even the largest Roman engagements were less than a quarter that: at Cannae, Paullus and Varro had 80,000, but most armies were even half THAT size; 40,000 at the Catalunian Fields, 35,000 at Zama, 42,000 at Carrhae; we are NOT talking about a comparably large amount of men here. I know the Chinese were famous for having put together immense armies, but there were 300,000 at Salsu, which is supposed to have been a truly epic amount of men in an army even for Chinese standards (though perhaps more typical of the Sui).
So I really don't know what you guys are talking about.