A new report puts some of the blame for Americans' expanding waistlines on the growth of new Wal-Mart supercenters in the US.

Big box retailers, and Wal-Mart in particular, have made cheap, bulk-size junk foods more readily available, and Americans are eating more as a result, argues the report, which was published by the National Bureau of Economic Research.

"We live in an environment with increasingly cheap and readily available junk food," Charles Courtemanche, an assistant professor of economics at Georgia State University and one of the report's co-authors, told the Washington Post. "We buy in bulk. We tend to have more food around. It takes more and more discipline and self-control to not let that influence your weight."

The researchers found higher rates of obesity in areas dense with supercenters, which have a larger selection of food and also offer other services, such as auto repair. Just one additional supercenter per 100,000 residents increases average body mass index in the area by 0.24 units and the obesity rate by 2.3% points, they found.

“These estimates imply that the proliferation of Wal-Mart Supercenters explains 10.5% of the rise in obesity since the late 1980s," researchers wrote.

The report was co-authored by researchers from Georgia State University, the University of Iowa, the University of Virginia and the University of Louisville.

The percentage of the population considered overweight rose from 43% to 67% between 1960 and 2006, while the obesity rate grew from 13% to 34%, the report says, citing statistics from the National Center of Health Statistics.

Meanwhile, the number of Wal-Mart supercenters has skyrocketed.

Here's how many supercenters there were in 1994, according to the report:

And here's what the map of supercenters looked like in 2009:

A Wal-Mart spokesman said the report doesn't take into account any of the company's initiatives to make healthy eating easier.

For example, the company is working towards reducing sodium and sugar levels by 25% and 10%, respectively, across all the products it sells by the end of 2015. Wal-Mart has also almost completely eliminated industrially produced trans fats from its products, the company says.

"Four years ago, Walmart unveiled a comprehensive effort to provide its customers with healthier and more affordable food choices," the company said. "Since then, the company has made significant progress in product reformulation, building stores in food deserts and providing access to fresh fruits and vegetables, as well as increasing charitable support for nutrition programs that help educate consumers about healthier food solutions and choices."

The spokesman also pointed out that Wal-Mart has donated more than $41 million to nutrition programs since 2011, including a $2.5 million grant to a program called Share Our Strength’s Cooking Matters, which helped more than 107,500 low-income individuals learn cooking and shopping skills.