Minecraft wiki entry for Spawner, associated text: For all of the volumes listed in the table, the horizontal plane is centered on the center of the spawner block. While the spawning volume for pigs is 8.9×2.9×8.9, the requirement of grass blocks that are necessary for pigs to spawn reduces the actual volume in which they successfully spawn to 8.9×1.0×8.9. Other mobs can spawn in mid-air, ignoring general rules about spawning on solid ground.