Usually large fish don't reach their natural size in captivity because of two main reasons, stunting and improper husbandry. Stunting is basically the reality of the old saying, "they will only get as big as their holding tank", which is true but their insides continue to grow like as in dwarfism, and then rupture and fail under pressure, usually bringing on stress and disease first. Improper husbandry isn't to say they're not being cared for in a good way, fed and given water changes etc, it's just any situation where we can't duplicate the details of their habitat, whatever it may be, usually a combo of many things, all adding up to a shorter lifespan.
There are plenty of experienced keepers of payara, for example, but I don't think anyone's gotten one to grow past a foot in captivity, I believe they get well over two feet in the wild. If memory serves me, some of these specimens were given ample room to grow, so it would be an issue of not being able to duplicate ideal environment that the fish and it's lifespan agrees with.
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