ASI decreases damage received by giants
This reduces damage take when another entity is not attacking you — essentially lava, fire, lightning, anvils, etc. The list is pretty large when you actualy get into it.
This option also causes beegs to resist damage completely below a certain amount due to their thick skin, so they just cant be hurt unless you deal enough damage. The threshold is one twentieth of their scale, so to hurt an 8X scale beeg you need to deal at least 0.4 damage in one hit (after armor and other reductions were taken into account!)
There are some exceptions, the following sources of damage cannot be resisted due to your size:
Starving in hunger
Drowning in water
Damage from falling into the void or world border
Damage from wither effect
Damage from /kill
Entity cramming
A special case is fall damage, since ASI does not affect it, but Pehkui does, so yes beegs do resist fall damage, but it is not ASI doing it.
By default, beegs will receive less damage and knockback the bigger their scale. This can be disabled in the Server Config:
actuallysize-server.toml
# This enables this feature in which beegs receive decreased damage
beegsAreTanky = true
# This disables this feature in which beegs receive decreased damage
beegsAreTanky = false
This option only applies when you are beeg, that is, greater than 1x scale. As you can see, the damage you take quickly approaches zero after around 6x scale.