It's still going to be the same process for assembly of the final product. The can doesn't get any quicker to seal just because the can of tuna is a little thinner. Thereby more assembly costs for the same amount of final product.
The four factors of production are:
Land (to include natural resources and raw materials)
Labor
Capital, and
Entrepreneurial ability (business savvy)
(Economics 101)
Reduce the cost of any and you've reduced the cost of production.
I'm talking about raw materials costs: smaller cans= reduced costs for raw materials. While the amount saved per can may be a minuscule amount, it really adds up when you multiply it hundreds of thousands of times.
Just ask Hershey's about their hundreths of an ounce savings on each candy bar. (And just BTW, I was in the supermarket just awhile ago and looked to verify: Hershey's is still listing their candy bar weights by the ounce and hundreths of an ounce, which produces some really bizarre numbers for a product net weight, I must say.)
You seem fixated on the "assembly" or processing part of it which would come under the "labor" aspect of production costs (although less so nowadays due to mechanization and robots).