of course, I may be stating the obvious here (which I do tend to do alot..) but part of the giant mouths is make up.
Watch an old black and white horror movie sometime, and you'll see that the people who have to scream and look scared have really dark lipstick on and thick eyeliner. This is to give the mouth and eyes the appearance of being bigger. Also, the lip line is covered up, and moved upward, so you're wearing lipstick about a few millimeters upwards over the lipline..and the mouth thusly look bigger.
This has spread into stage makeup also. If you look at the pictures of Idina, she usually has on really dark lipstick. She does have a huge freakin' mouth, but I'm just saying is all.
Was that a fat joke?