During hostile mob spawning (I have not witnessed this issue for Magma Cube Mob Spawners), the largest size Magma cubes can spawn inside of blocks. After killing one the smaller variants come out of the blocks. I have seen them spawn in transparent and solid blocks alike and even heard them suffocate inside of walls. I assume this is because their required spawning space is smaller than their hit/hurt box. When they do spawn inside of blocks, I witness it most often inside of a 2x2x2 space. I play exclusively on Realms, not sure if this happens also on Single Player. See attached image: