Can sizes today seem like they change constantly. I remember when tuna came in 6 3/4 ounce cans, more recently the cans were 6 ounces, and now they are just 5 ounces. Similarly, I remember when commercially-canned peaches were in 1 pound (16 ounce) cans; now the cans are only 15 ounces.
A hundred years ago there were standard can sizes, and people often referred to cans by their size number. For example, I’ve seen old recipes which call for 1 – No. 3 can of tomatoes. There actually still are standard can sizes, but the size numbers aren’t something on the tip of consumers’ tongues like they once were.