Gallic appearance

[Aryaman2]

So why would the 'wild' Germanics be taller than the 'civilized' ones? Because it fits the poet, that's why. It would be prepostrous to even assume that Germanics shrunk upon becoming civilized.. D

There is an anthropometric study in ancient Raetia that shows just that, average size of people diminishing after Roman conquest. That is not to deny the particular example given by Ammianus being a poetic convention, I am myself convinced that is the case in many ancient sources, but it seems that Roman social system being more unequal than "barbarian" system actually have the effect of lowering the average size of population

Aditional thoughts on the subject
I recall a passage of Orosius in which Roman legionaires are scared of the size of tall Lusitanian warriors, that looks like a literary tropos, after all modern portuguese are hardly among the tallest Europeans.
OTOH I recall a passage of Caesar, I think it is in the siege of Avaricum, in which Gauls mock Roman legionaires, because they are so short they will not be able to move the large siege machines they had built. This comment however rings true, doesn´t look like the literary tropos but like a personal anecdote. I have posted before how Gallic warriors as a class were taller than average Gallic population, and I think that Roman legionaires of Caesar time would be mainly lower class, Wellington´s scum of earth, so it is possible they were shorter than average. I have read some statistics of European conscripts in XIX century and they were really short, suffering all kind of undernutrition evidence, for instance the average Danish recruit in mid XIX century was just 1.68 mts, while the average Spaniard was a diminutive 1.57 mts. So after all, while a literary tropos, could have some true in it.