The US has some extremely wealthy people who pull the average way up.
We also have more people in poverty and near poverty than most developed nations.
The site you linked to says that the US has the third greatest income inequality of any developed nation - after only South Africa and Mexico.
That link doesn’t have a link to rankings for GDP per capita. I think you’re looking at the link to GDP. You’re right about the link being inadequate to making the point – GDP per capita has never been a point of controversial point in this thread.
In the context of this discussion, GDP per capita doesn’t really enter into it. How many people have the means to eat well, and how many are mainly consuming a diet of cheap processed food?
There are other metrics by which Americans can be consider wealthier than people in other nations. Car ownership, for example, is much more common in the U.S. – even for people who are considered impoverished. In many areas, housing costs are at levels that are unthinkable compared to other countries. Spending more money on rent doesn’t mean you’re “wealthier,” though.
The U.S. has a much larger percentage of people living in relative poverty, and that’s going to effect how they eat – particularly as good food is in large part considered a “luxury” item. (In Canada, too.)
Don’t let nationalistic pride distract you from what we’re actually talking about-- How many people can afford to eat well vs. how many people settle for cheap convenience food with little actual value.
I would be interested in seeing some statistics about relative average incomes (adjusted for purchasing power) for different nations, too. I wonder why they’re so hard to find?
Plan B, and others interested in overall poverty, take a look at the current NY times series on social class. The link I’ve posted is to an interactive graphic thingy which compares social class mobility in the U.S. and several other countries. It indicates that generational class mobility is slower in the U.S. than in France or Canada-- but it’s still fairly good.
However more delving through that graphic shows that if you’re poor now, you’re still going to be poor in 10 or 20 years.
Obesity appears to correlate with poverty and lack of education. It is my opinion that Canada fares much better than the US on that score for the general population. That is not to dispute that Americans on the whole are wealthier, have some superior educational institutions.
On another note, we recently had a family gathering that included relatives from Texas here in British Columbia. In preparation for the dinner, these relatives spent several hours trying to find a local butcher that could supply them with a cut of meat call brisket. Finally they came home with about 20 lbs for 25 people and began to prepare it Texas style for the dinner the next day.
Those Texans really know how to prepare meat! Just about everyone came back for seconds and thirds.
I should add however that the Texas relatives were rather thin compared to the Canadian relatives.
Obesity is perhaps more the results of what people eat at home rather than in restaurants. The majority of people eat at home where they can raid their refrigerators any time between meals.
There’s a study out of the Mayo Clinic indicating that minor daily activity is a better indicator of overall body shape than diet. Maybe Canucks just fidget more 
Or shiver more. Igloos can’t be that warm.
Is it possible to compare obesity rates between two countries and factor out the socioeconomic differences? If you compare Canadians to Americans with similar professions and socioeconomic backgrounds, is there that much difference in obesity?
Of the world’s trillion dollar economies (US, China, Japan, India, Germany, UK, France, Italy, Brazil, Russia, Canada and Mexico), Canada’s GDP per capita is second only to that of the US. Given that other countries in the list as cited above as having obesity rates similar to the US (or, at least, much higher than Canada), your conjecture doesn’t seem to be correct.