I'm saying the average weight of a wild tang compared to a captive is a lot higher then when compared to other fish. Which means that either the lack of space or the diet is having a greater effect on the health than it is other fish. Now predation is a vital part of keeping a population healthy and while that may explain why captive species are more likely to be unhealthy it does not explain why our tangs living the easy life fail to reach the health of wild species.That's also a little bit of confirmation bias. The big fat giant tangs you see in the wild are the exception, not the norm - they spawn in the thousands and very few survive to that age. Of those that do, not all of them will ever reach that size even in the wild. Size variation in our own species is as high as 4x between healthy individuals. We wouldn't claim someone is malnourished because they aren't Hafþór Julius Bjornsson. That 'max size' is just that. It's like listing the max height of a human at 7'6" because Yao Ming exists. It should be a bell curve. Like a yellow tang can get to 8 inches long but on average the peak of the bell curve is more like 6 inches.
I oppose this to groups of other fish like damsels and wrasse which from my observations are often much fatter in captivity than in the wild.
Predominantly herbivorous animals can develop issues from eating a protein based diet. It depends on their digestive system and I'm no fish expert but symptoms can range from constipation and/or diarrhea to blood being poisoned with high levels of ammonia or urea. From looking at threads on here it seems to cause constipation and lack of appetite sometimes leading to starvation in tangs and rabbitfish.But I'm sure diet helps too. They are definitely getting what they need from a meat-rich diet but perhaps with a primarily veggie diet they grow slower but also grow for much longer?