This is a common misconception due to the fact that the bun for the Big Mac was a lot bigger back in the day. In fact, Big Macs were made with the smaller hamburger patties well before Supersize Me. I worked there in the late 90s and we used the smaller hamburger patties back then. My mom worked there when I was a kid in the early 80s and they were using the smaller patties then, too. In my lifetime, I've never seen a Big Mac made with anything other than the small hamburger patties. And just to check myself, when looking online, I can't find any references to the burger patties being bigger going all the way back to its launch in 1967, while some sources state that the burger has been mostly unchanged, aside from the bun size and the recipe for the Mac sauce, for its entire existence.