I think you are confusing canned tuna with canned sardines. If not then different brands maybe.
Are you getting the same kind of tuna? Chunk light vs albacore or yellowfin, or oil packed vs water?
The cans are a lot smaller than they used to be. Pre-2000 they were 7 ounces, now they are 5 ounces, though some are as small as 3.
Once upon a time "chunk" light tuna was actually chunky. Now it looks more like cat food.