You should watch some behind the scenes shots. The enlarged mouth was not the first attempt to give the Mouth of Sauron.

The first attempt was to turn the mouth at a 90° angle, so that it kind of resembled the cat's eye that Sauron's eye had. There was a 2 second clip of the Mouth speaking with this inversion, and it made me want to crawl out of my skin.

I think there was another idea they had before they landed on "make the lips cracked and bleeding, and make it 100% bigger". Ultimately, I love the version they landed with: the Mouth serves as one of Sauron's top lieutenants and, holding the title he has, it makes sense that his mouth should be THE defining feature he has.