Antesignani?

[Watchman]

Eh, it's actually a question of local ecology, climate, population density etc. For example a 16th-century upper-class French traveler observed in his diary that a full Mediterranean meal roughly amounted to what was regarded as appetizers north of the Alps...
Plus, animal protein - a key "building block" when size growth is concerned - was always in a particularly short supply around the Med. Not too much to hunt, not much to fish in the sea, not that good conditions for raising pigs and cattle for slaughter... Conversely, north of the Alps there was far more of all of that (particularly fish, on the shores of the Atlantic and the Baltic) for much more dispersed populations.

Also, recall that the defense skill score does not entirely stem from out-and-out ability to ward off blows with your shield and weapon, but also general readiness and ability to simply get out of the way of attacks - something that's generally easier when not weighed down by armour. And also something you're going to pay that much more keen attention to when there's nothing to stop anything that connects...

Mind you, not every unit that under this logic was given the "agility" bonus due to considerations of inter-unit balance. Bataroas (the helmeted, shirt-wearing Gallic longsword guys) are and example, as the bonus would've driven their defense score total to a number unecceptably high for their price range and compared to some other, better equipped and hence more expensive, Celtic units. Botroas (the spiky-haired, topless longsword dudes) on the other hand duly got it.