Minecraft wiki entry for Magma_Cube, associated text: A magma cube needs roughly a 3×2.1x3 free space to spawn. The area in which the magma cube spawns in must be clear of solid or liquid obstructions. When a magma cube attempts to spawn, the game checks for the space requirement of a large magma cube, and the size is determined later. Therefore, since large magma cubes are slightly taller than 2 blocks, no magma cubes, regardless of size, are able to spawn at all in 2-block-high areas. Also, since large magma cubes are slightly wider than 2 blocks, and mobs spawn at the center of a block, having any block within the 3×2.1x3 area, even as thin as a glass pane, can prevent a magma cube of any size from spawning. These conditions are in effect since the Nether Update.