Minecraft wiki entry for Defeating_a_bastion_remnant, associated text: This disabling grid makes use of the mechanic that a magma cube, no matter large, medium or small, checks for the space requirements of a large magma cube before it spawns. Since a large magma cube is slightly wider than 2 blocks, there is no horizontal space within the 9×3x9 area for it to fit in. Also, a spawner can only spawn a mob's "feet" at 3 heights: one block below the spawner, same height as the spawner, or one block above the spawner. No matter which layer it chooses, even the lowest layer, since a large magma cube is slightly taller than 2 blocks, its "head" will slightly intersect with at least one of the solid blocks, cancelling the spawn attempt.