There are many factors people seem to ignore when discussing looting or fishing. The reason you see many people get good loot from Sand Tigers is because they are very frequently targeted and prioritized over other species. This is most likely because they have a very high gold reward and a decent rep reward. So if someone catches one species more than any other they are of course going to get many good loot items from it, and then they post screenshots and tell other people that they got good items from x species, so those people target that species and prioritize it, and of course get good loot, and the cycle continues. I can't say with 100% certainty that all big fish have the same drop rates, but once I take all other factors into consideration, I can't see any noticeable difference in them.