Even if it's technically the most common numerically (after all, it takes more men to make larger formations), what of employment? The most "common" unit numerically in modern warfare is a fireteam, but I doubt that they'd march independently as often as, say, a company.
A Cohors Reformata would be a cohort of 480 post-Marian legionaries, and was, IIRC, the predominant military formation of the Roman legion. Whereas, you can put Polybian/Camillan units to larger sizes due to the way Hastati, Principes and Triarii were historically arranged. We can get a rough idea of barbarian formations from historical accounts, numbers and populations at the time, along with looking at foreign influence from Hellenes.
It could, technically, lead to smaller battles in some cases, but that's because the IG numbers required to get, say, a legion are reduced to make for both a more realistic game (where you can field legions without the use of 0-turn or scripting) and allow the construction of even larger forces, like consular armies, and give a proper feel to battles involving rare/advanced units, which otherwise would make you ask "Why is the Sacred Band/Praetorians/Consul/General showing up at a minor skirmish?".
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