Actually according to this article (apology in advance for posting the Daily Beast liberal trash) canned tuna back 20 years ago was actually 7 ounces, then shrunk to 6.5, then 6.125, then 6 and then 5, all the while the price stays the same or goes UP even with the downsizing.
Article from 2010 documenting tuna can shrinkage from 6oz. to 5oz.