Shield Bash Damage
The winner of the crash is determined by weight class, but the damage is determined by the total integrity (or sum of barrier and integrity) of the winner. Something around 10% of the total integrity of the winner is dealt in damage to the crashing target.
Given the massive HP difference between tanks and mechs, that’s why stepping on them usually breaks threads instantly if now outright destroys the tank.
Note that actual damage dealt is a bit hard to predict because multiple collisions can occur. If you push an enemy up a slope or you just keep dragging them against the floor, they can take multiple hits.