Big and Medium size Slimes have their own hitboxes, but when I killed a Medium Slime, they were splitted and got pushed further than I excpected.
The Bug here is that the Small Slime Hitbox are somehow The Big Slime Hitbox, though having the Debug to check the Hitbox, the size for it is normal for the Small Slime.

The Screenshot below is shown the Small Slime are on 1 each Grass Block when they are suppose to move instead of being big. The 1 without the hitboxes, the other 1 with Hitboxes on are shown aswell.

A video is also shown with Small Slime's Blue line (The Direction of what the Mob is looking at) has an interesting hitbox when it comes to Bows/Crossbows. You are able to hit him with the arrow, but not able to hit it with the sword.

(Edit: This also affects Magma Cubes since they are similar to Slimes, but since they jump higher, they move pretty rapidly).

That's the Bug I got for you today. Have a nice weekend!
-Th0mas