That's an interesting read. I have a suspicion that if we are seeing obese fish, then diet has more to do with it than anything. Aquafarmed fish were mentioned as the obese ones in that link, above ^^^^ and I would be willing to bet that the food they were being raisedon was some type of super high-fat, high-carb diet that is designed to bulk them up as quick as possible, as opposed to a natural diet like what they would ordinarily eat. Same reason grass-fed cows are smaller than grain-fed ones, but their meat is way healthier to eat.
I also agree with the poster in there who sais that his fattest fish were always his longest lived. I have never owned a fish I would consider obese but I think catfich, for example, with a big gut is a normal thing. Ditto on puffers. Probably 3/4's of the photos of Mbu (not so much fahaka) I have seen from captive fish keepers, I would consider underweight.
Getting back to the topic at hand, I don't believe you can feed a big puffer too many crawfish. That's a super-nutritious food item that is, unlike just the tail of a prawn or the leg of a crab, nutritionally complete and has no artificially manipulated leves of fat or carbs.
As always, just my 2-cents.