I don't know how many of you have noticed, but tuna now comes in 5 oz. cans, not 6. The price didn't go down either! That's a 17% decrease in the contents!
To stretch it further in sandwiches, I have a number of favorite add-ins: a chopped up hard-boiled egg, diced apples (I use Gala, because they're a little sweeter, and really cut the "fishy-ness" of tuna), celery, and even shredded carrots!
I add half a small-ish apple to one can of tuna, a big sprinkle of carrots, and/or one egg per 5 oz. can used. I also treat the celery differently than you'd think.
I *hate* soupy tuna, and celery is mostly water. I chop up the celery, then wrap it up tight in a clean cloth or paper towel, and squeeze it hard. This gets a lot of the water out so that, if I use only part of the tuna in a day, the next day's batch will be just like I like it.
Another idea is a handful of cooked and drained fine egg noodles mixed in as an extender. I was served a tuna salad sandwich using this idea by a friend.
I chop a large Vlasic zesty dill pickle or two, squeeze out the juice because I can't stand soupy tuna either, and add fresh diced onions or dried minced onions :-) Yummy :-)
Years ago I ate some tuna salad I loved and asked the girl who made it what she did different. She adds crushed up saltine crackers along with the eggs, pickle, mayo, etc.
Years ago, a friend of mine worked in a deli. He told me that they used to stretch the tuna by adding plain white breadcrumbs to it - I had eaten the tuna there many times before and thought it was just pure tuna and mayo - I had no idea the breadcrumbs were there!