Tuna "grows to a length of 14 feet (4 meters) and a weight of 1,800 pounds (800 kilograms). Its average weight varies from 60 to 200 pounds (25 to 90 kilograms) or more," according to http://www.britannica.com/EBchecked/topic/608882/t...
Forget about the 1800 pound tuna for a moment.
If the "average tuna" weighs upwards of 200 pounds, can you imagine how large the can would have to be to fit that fish? You would have to seal it in a can the size of a truck gas tank at minimum.
Store shelves are not designed to hold cans that size. Can openers would have to be the size of the Jaws of Life to get a can that size opened.
Furthermore, once you crack that can open, you will have to eat a lot of fish fast! Or have a big tuna party. Otherwise much of it will go to waste.
Tuna is packed in portion sized cans for good reasons my friend :)