Tuna in a Can

Has anyone wondered why a can of tuna is more like a half-can of water or oil and a half-can of tuna mush? The can still advertises tuna IN water or oil. I can't even count on making two sandwiches with a single can, even if I add lots of extra stuff, like mayo, onions, relish, celery, etc.

Of course, you can buy solid albacore for 3x times the price, but you give yourself 3 times the mercury as well. Such a deal.