I see what you mean from a gameplay perspective, but is there any significant evidence that the "barbarians" of northern Europe were consistently able to mobilize significantly larger armies than their more populous Mediterranean counterparts? I'm not talking inflated Dei Bello Gallico numbers here...
On further reflection, if the barbarian factions were ever able to unite into a larger confederation as represented by conquering a large empire in the game, i suppose they would be able to mobilize a larger force from the various tribes... this would work for the levy units in the game (which are already as large as a phalanx if i am not mistaken), but could be unrealistic and overpowered for elites such as the Solduros.
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