Restaurant portion sizes have exploded in recent decades. A fast food soda is six times bigger now, than it was 60 years ago. Burger size has grown threefold.

A new report from the Hudson Institute picks up on a more recent, countervailing trend: Fast food restaurants are gravitating towards lower calories options - and customers are buying them.

The report looked at the offerings in 21 chain restaurants, ranging from Burger King to Panera Bread in 2006 and 2011. "Low-calorie" offerings were defined, for an entree as fewer than 500 calories. For a dessert, anything less than 150 calories made the cut.

When the researchers looked at the number of servings at all 21 chains, they found an overall decline of 832 million items ordered. That probably had a lot to do with the recession, and fewer Americans eating outside their homes.

At the same time, the number of orders for lower-calorie items jumped by 472 million. Orders for traditional, higher calorie items fell by 1.3 billion, as you can see in this chart below.

It's one thing for restaurants to offer lower-calorie dishes, it's quite another, however, to get consumers to buy them. On that front, the Hudson Institute did find a bit of evidence of consumers gravitating toward less caloric items. French fries, for example, made up a slightly smaller share of fast food sales in 2011 than they did five years prior.

Lower calorie beverages, meanwhile, saw a 9.5 percent increase in sales, compared to traditional sodas, which grew by 1.6 percent.

The health care law will soon require all chain restaurants to post calorie counts on their menus (final regulations are still in the works). That could put a greater emphasis on creating healthier choices.

Still, let us not forget that high-calorie offerings aren't exactly difficult to find. The Center for Science in the Public Interest recently complied calorie restaurant dishes. It had no trouble finding multiple offerings that weigh in above 2,500 calories - as you can see in this chart below.

its annual list of highest-