While I know the others before me have said it very well, i'll reiterate for fun
I did the exact same thing with a hippo tang and I didn't listen. The fish was the size of a dime, I figured it'd be happy enough. I should have taken that fish back immediately. I didn't. That fish died and took my whole tank down with it after it got ich from the stress. I have respected tank sizing ever since. Just because the fish doesn't show visible signs of being cramped, doesn't mean he doesn't feel it. That's one of the bigger tangs, he needs even more room than most.