Ok, I come back with some facts now! Based on observation.
It turns out that damage output by enemy squads (I havent checked yet if this applies to player' squads too) doesn't decrease at the same rate that squad number do. So, for example -and this is not theoretical, but my notes on an actual combat-, an enemy squad of 18 thieves had 107-145 damage. Numbers are good so far: 1 thief does 6-8 damage, so 6-8 damage x18 = 107-145. Cool. BUT WAIT. I kill 13 of those thieves, so now the squad is down to 5 thieves. That should amount to a damage of 30-48, because 6-8 x5 = 30-48. Well, guess what? Their display shows a damage value of 65-89. This happens consistently across several fights. Both with thieves and archers.
Now let's go try this out on Steilings, just for fun. I face an enemy steiling squad of 4 members, that's 53-139 damage, 13-35 x4 members, ok. I kill 3 of them, so that squad is down to 1 member. That 1 member has 26-70 damage.
Surprisingly, this doesn't seem to apply on strike-back attacks. There was this squad of 31 thieves (184-250 damage), I killed 19 on an attack by Axe Barbarians so the squad went down to 12 members. Inmediately they retaliate against my Axe Barbarians, doing a meager 39 damage. This happened several other times, although I did not took down notes about actual projected damage outputs. Must pay attention in the future.
And almost everything 1 shots ranged units