Realized that some clarification was needed about a "can" of salmon... That would be a tall can, or two short (tuna or catfood) sized cans. That would be roughly 15 oz., I think years ago the standard can of salmon was a full 16 oz.
Shrinkflation where the sizes have been made smaller for more profit/same price have made some difference in old recipes. Some things are still close enough, but there are a lot more sizes now and they aren't always filled with the same amount of ingredients (more liquid for example).