It takes only one layer of Obsidian to completely protect a player from TNT explosion damage but it takes 5 layers for 3 block tall mobs, 6 layers for 2 block tall mobs, 7 layers for 1 block tall mobs, and 8 layers for paintings and item frames. Now replace all Obsidian layers with Glass and it will take the same amount of layers. This works with all explosion sources (but they need different amount of layers of course). This means that explosion damage ignores all blocks when it hits mobs. I had a theory that maybe it doesn't cause damage but just plays the hurt animation but no, it deals damage.