I first noticed this while testing slime barriers for farmland. After a significant amount of small slimes were added, they began to push one another into a grid. Each slime had a few blocks to roam, and should not have pushed each other like they did if they had the proper hitbox. Attached is an image of my first notice and of a subsequent test. I have not tested this with any other slime size. Relogging has not fixed the issue, nor has a game restart. I have only tested this in the latest snapshot (19w12b).