Game difficulty hostile mobs spawning

I spawned large and small slimes. Then I set the game difficulty to easiest or so called "peaceful" and the larger slimes despawned BUT the smaller ones remained. Aren't small slimes hostile mobs to? I am also able to spawn creepers, zombies, skeletons etc in difficulty mode peaceful, switching back and forth has no effect on their part; either way they stay spawned and do not despawn. All hostile mobs should despawn when the game difficulty is set to peaceful do you agree? For example you already despawn the larger slimes on a certain difficulty mode, I would just do that with the rest of the hostile mobs that are the same difficulty etc. Otherwise the difficulty setting isn't very useful.