Is this due to being in a bigger tank? This is a different tang than the OP was taking about though.
No. Young fish grow very fast.
As someone who use to keep a lot of Koi, large fish tend to do a lot of their growth in their early years, but not all species of fish may follow this trend, and really slow off or completely stop growth later on. My koi during the first two years could hit upwards of 18 inches and if I was lucky even 22"-23".
You can grow massive fish in a small tank. I've done it, countless times.
The truth is that most people don't feed enough during the growing stages. For example, koi, you need to feed 3% body weight daily, continuously, for 3 years. Meaning as the fish gets bigger, you need to increase the amount it gets to eat. After the third year, growth, for the majority of koi, growth literally slows down to a turtle pace.
If you have corals and are trying to keep nutrients low. Early on, your feedings are enough to grow small specimens relatively quick but most people don't scale the food to the increase of the fish's size and needs. It'll hit a certain size and require even more food to grow but since reefers limit their food source to maintain water quality, growth potential becomes very limited, especially if you have a ton of other fish to compete for food. Captive specimens don't get to eat whenever they are hungry like wild specimens. They get to eat when you feed them and if you don't feed them enough, they most likely will not hit dinner plate sizes.
I have a 4-5 year old yellow tang in my 45 gallon shallow. He came in to my LFS bone thin. He was near death and I nurtured him back to health. However, because of the malnutrition during his early years, he's not grown at all. It took 4 months or so before he finally stopped looking like a skeleton tang. Little guy is ~4". Most folks think he's juvenile... lol.
If you purchase a 2-3 year old tang that's 7 inches. It's not likely to put on another 10-12 inches for you for the following year or years to come. Good water quality and feeding may give you a few more inches but nothing like what you'll see a young juvenile fish will put on in just a few months.
If you plan to do a lot of feeding in your systems, you can potentially grow a very large tang if you pick up a healthy juvenile and should be concerned about getting one for a smaller tank but if you are feeding the bare minimal to maintain water quality, you're not likely to produce dinner size plate tangs.
A pinch of flakes, three times a day, isn't going to produce a 2-4 foot koi...
I don't see very many wild size specimens in captivity. Most are much smaller than their wild siblings.
