The collision box of the medium magma cube is smaller than it should be, making them harder to hit. From the entity json files:

Not sure if this is a glitch or not. But while fighting magmas they have become harder to hit while going down in size. Not sure if it's meant to be like that or if their hit boxes are just that small and annoying to hit. I have died multiple times while fighting these things. https://youtu.be/1a2Db8FDbYo