I was exposed to this myth in my youth as various European nations described their misadventures in the US of A or encounters with Americans and as a result came to US with wrong expectations. Now that I live here, things a little more clear to me. It is a nation of more than 350 million people ( depending on how and who you count ). Even a small percentage of certifiable idiots will be very well represented in terms of absolute numbers.

I would not say Americans in general are dumb though. I would say that:

1) they are under-educated ( and then we can also get about the quality of education for those that were educated ) 2) they are very heavily propagandized

Even more than that, the accounts we get claim that some of the people in the focus group said the 1/3 pound burger was smaller than the quarter pounder. Even if those claims are accurate, it's possible they were looking at the diameter, since the 1/3 pounder has two smaller patties stacked on top of each other, while the quarter pounder is one large patty.

Most places that report on this say that the third pounder was cheaper and that Americans preferred the taste, but they didn't buy it because of the name and their poor math skills. But its name was changed to "Papa Burger," and it still didn't become the quarter pounder killer it's portrayed as.

And since we don't know the context they could have shown a closer up burger, or heck, taller burger that is 1/4 and a 1/3 next to each other but further away or stepped on at which point they were asked, what's bigger?

Also, the 1/4 or 1/3 is referring to the raw, uncooked meat only. So in theory you can add 1400 lbs of lettuce to make something larger than my car. (Which is terrible, since lettuce is where you pick up all the disease these days - everyone here should order all their food without any lettuce)