In MTW, some units wouldn't change their size, regardless of the size setting. This included general bodyguards and siege engines (and i think that in STW, it was true for the kensai).
The question is, do you think it would be necessary (or at least convenient) to do this for siege engines in Rome?? Because i just don't see any good reason 64 men are required to handle a couple of ballistas, especially when 16 can do the job perfectly well.
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