Tuna Fish Scam!

I have asked this question for YEARS and no one can give me a definitive answer:
(Costs are approximate)
1) One 6 oz. can of tuna fish costs 80 cents.
2) Two 6 oz. cans of tuna fish cost $1.60.
3) One 12 oz. can of tuna fish costs $2.69.

So why isn't the 12 oz. can either the same price or cheaper than the two 6 oz. cans? What are we paying for? Extra can material? If anything, the larger size should be the smarter, "economical" purchase. These companies are relying on the fact that we Americans are just plain stupid. Will someone finally shed some light on this???