Here's why:
* Fish are mostly water: Their bodies are about 70-80% water, which means the water inside them makes up the largest percentage of their weight.
* Bones are lighter than water: While fish do have bones, they are generally lighter and less dense than water.
* Muscle is denser than water: Fish muscle is denser than water, but even so, it's still less than the total amount of water within the fish.
So, even though a fish's muscle or bones might seem like the heaviest part, it's actually the water inside that contributes the most to its overall weight.