The shadow under Slimes changes to match their different sizes, but a Magma Cube's shadow is always tiny, being barely visible for bigger sizes.

Steps to reproduce: 

• Summon a few slimes of different sizes
• Summon a few magma cubes of different sizes
• Notice how the slime's shadow is based on its size, while the magma cube's is not

This can be easily seen by summoning a Magma Cube with no AI, slightly above the ground, with the command /summon minecraft:magma_cube ~ ~ ~ {NoAI:1,Size:2}