Large Magma Cubes will spawn in a 1x2x2 space which should be too small for them to spawn.

I have a Fortress mob spawner setup. The spawning location is two Nether Brick Blocks, a block of air above each one, and a block of Glass above each block of air. One space into the negative on the X-axis from each Nether Brick Block is a block of air. That's a 2x2x2 space total with only two spawnable blocks in it. The spawning area is surrounded by Nether Brick Blocks. I stand 28 blocks below the spawning location and up to two blocks negative on the X-axis while waiting for mobs to spawn. They spawn as expected until a large Magma cube spawns which gets stuck in the spawning location and prevents other mobs from spawning.