Even in the ocean few tangs I have seen are "dinner plate size".
No surprise that in our tanks they are the rare exception.
False. I have dove in Florida and the Caribbean and snorkeled in Hawaii, the Sea of Cortez, and the Galapagos, and tangs get big! Its crazy seeing large Atlantic Blue tangs that are 12"+ and 2"+ thick, and literally dinner plate sized yellow tangs with their dorsal and anal fins stretched.
I personally think our fish get stunted in our tanks. Whether it is a build up of nutrients or hormones or some other factor. Also I wonder if it could be because fish have to forage for food in the wild, where in our tanks they can be lazy all day especially when they know the feeding schedule.