Minecraft wiki entry for Cave_Spider, associated text: While a cave spider's hitbox is only 13.9% the volume of a normal spider's, its model's volume is 34.3% as large. The shadow is even larger: compared to the normal spider's shadow it has an area of 49%.
This is the only mob that can be instantly killed by a direct hit from a tier 2 Splash potion of harming (Instant Damage II) due to its health being the same as the damage dealt.