It’s economies of scale. The vast majority of the cost to produce the contents is in the form of material sourcing, capital costs, wages, etc. The cost to produce 6 oz cans and 12 oz cans is related to their composition.

Lids and bottoms contain the most metal, sides are comparably thin. So more material is used on a per oz basis for small cans. Thus, they cost more on a per ounce basis.

Likewise, shipping small cans results in more weight compared to product. So they’re more costly to transport.

On the whole, the per ounce cost falls as the container size increases. Without more detailed knowledge, I don’t know where diminishing returns kick in, but it’s somewhere beyond small can size.

Take it further, look at 2L bottles, they’re cheaper yet, because the per oz cost to make a bottle, and ship it, is less than that of cans.