Video: https://www.youtube.com/watch?v=2sLMhqhL2zo

Most mobs will only register a hit on the player after a couple milliseconds, leaving enough room for the player to get away (usually). It acts almost like "invincibility frames" in other games.

For some reason the Big Slime (and only big slime) seem to register a hit on the player almost immediately. Making large slimes a much bigger threat. The attack damage is not the issue here, only the timing between attacks being registered.