Considering certain thinks - here enemy numbers - as "impossible" does not mean that the author in question has to be a pathological liar in general. Numbers in particular is something that has to be handled with care when dealing with ancient sources of any kind. The best example is always Caesar: no one can expect his report to be the truth and nothing but the truth. That does not mean that we cannot take anything from him.
Things are bit more complex: It took the entire Age of Reason to reach the level of organization where operating with armies that were larger than what could have been moved and supplied for longer under the given technical situation became possible. From they Ancient armies only the Romans seemed to have achieved that level of organization (and you won't find any serious historian, regardless of which century, that wouldn't agree the Roman army was on a very high level of organization). But Roman authors claim that their army where the only one that did not operate with extra-hughe armies.Reasoning that "level of organisation that was needed wasn't reached until age of reason" shows arrogance of evolutionist thinking XIXth century scholars, who believed that all things developed to reach peak of civilisation it their time.
In return this would mean that either the organization was in no way unique or superior to her opponents, even that Rome's enemies were in fact much better organizied than SPQR (and that would be re-writing history), or that the enemies' numbers (in particular for lesser origanized 'Barbarians') given by the Romans were an exaggeration.
A hint to the real numbers can be in fact the numbers the Romans fielded. We know that they had the potential to field even larger armies under serious pressure (like Cannae) but usually didn't do so. Most logical would be, that they didn't do so because larger armies in this and that theatre in question had not been needed. When we redruce the numbers in the Barbarian armies to "about or a bit under" the number of Romans raised against them, they would perfectly fit into everything else we know about army sizes in Ancient history (numbers from Greek world for example).
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