Magma Cubes are Deadly

Put the summary of the bug you're having here

What I expected to happen was...:
Survive the large magma cubes

What actually happened was...:
Magma cubes hitting me trough blocks and knocking me away 3 blocks away

Steps to Reproduce:
1. Go to the nether
2. Find a large magma cube
3. Box yourself in with blocks 
4. Wait for the magma cube to hit you