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.
If this indeed occurs in the b snapshot, MC-267927 should be reopened. Your screenshots say a, but your report says b...
Duplicate of MC-267927.