The bug

Based on the code slimes with a Size value of 0 are probably not supposed to despawn. This makes sense because they cannot damage the player.

Note that this does not apply to magma cubes since they can always damage the player, even when their Size is 0.

Reproduction steps

1. Summon a small slime

   /summon minecraft:slime ~ ~ ~ {Size:0}
   

2. Change the difficult to Peaceful
   The slime despawns

Code analysis

The following is based on a decompiled version of Minecraft 1.17.1-pre1 with Mojang names

The reason for this is that the method net.minecraft.world.entity.monster.Slime.readAdditionalSaveData(CompoundTag) adds 1 to the read Size value before storing its value as net.minecraft.world.entity.monster.Slime.ID_SIZE (and the value is clamped between 1 and 127, both inclusive).

However the method net.minecraft.world.entity.monster.Slime.shouldDespawnInPeaceful() tests if the value is greater than 0, which is therefore always the case.

Instead that method should possibly call isDealsDamage() to determine whether the slime should despawn (though this would allow NoAI slimes to remain in the world even if they would normally, with AI, would damage players).