Small Slimes with Damage Attributes do not deal damage, nor do they even with Strength effects.

To recreate, do /summon slime ~ ~ ~ {}
This will spawn the smallest size of slime

Then while standing next to the slime do /attribute @e[type=slime,limit=1,sort=nearest] minecraft:generic.attack_damage base set 100

Go into survival mode and notice how the slime doesn't deal damage.

You can also do /effect give @e[type=slime,limit=1,sort=nearest] strength infinite 25 and it is still unable to deal damage
 

(Spawning a Slime with damage attributes already also does not effect anything)