How do trample mechanics work?

[Tuidjy]

Trample is most definitely screwed up. It seems OK when it is Elephants against
infantry, much less so when it is an ice elemental against an Jarl, and way off
when it's Troglodites against skirmisher cavalry.

For starters, a successful evade should result in minor fatigue, not real
damage. Second, the chance of evasion should depend on the AP difference
between the two units. And third, the damage should depend on the size
difference, not just on the size of the trampler.