There’s always a bigger fish

For Bedrock edition since salmon already has different sizes, (or java edition too when they add parity).

The chances of a small one is the same but for the bigger ones, for every salmon that spawn it has a rather 10% chance of it being 1.25x larger(adds 0.125 blocks in size), and that salmon also has a 10% chance of adding another 0.125 to the original size (1.5x the normal salmon) making it a 1% chance of seeing a salmon 1.5x as large. The size limit is when it reaches 10x (which is so rare it’s unnecessary). The chances get so low so quickly!

The chances are like that of a chance an ender eye is placed in a portal frame naturally, so the chance of a salmon being only 3x as large is 1 in a trillion, which is so incredibly rare already. The feature doesn’t have to be useful, it can be a nice little easter egg just for the phrase “There’s always a bigger fish”, Although you can bucket a large salmon and put it in your aquarium to impress your friends with it.