The bug

All mobs require two non-full blocks to be spawned naturally even if the mob is only one block tall.

How to reproduce

1. Create a Superflat world with the following preset

   minecraft:stone,minecraft:air,minecraft:stone,minecraft:stone_slab
   

2. Use for example the /say command to list all entities

   /say @e[type=!player]
   

   → No entity is listed

3. Replace some blocks with non-full cube blocks like slabs

   /fill ~ ~-1 ~ ~170 ~-1 ~170 stone_slab
   

   Note how the available space is still one air block, but now the block above it is a non-full block (stone slab)

4. Use the /say command again

   /say @e[type=!player]
   

   → Bats and spiders are listed

Code analysis

Based on 1.11.2 decompiled using MCP 9.35 rc1

The method net.minecraft.world.WorldEntitySpawner.canCreatureTypeSpawnAtLocation(SpawnPlacementType, World, BlockPos) tests for all mobs for two non-full cube blocks, regardless of the mob size. The same applies to water mobs in this method as well.