Magma cubes have too big hitboxes. It appears that their hitboxes vary depending on their type/size (small/medium/big) but any size has a hitbox a few times biiger than them.

Particles go out of their hitbox size, not their "real" size, and it is surely a real hitbox because it is punchable and they sit on the block few blocks way from them (so they are in the air).

Other mobs have regular hitboxes.