Do super-sized supermarkets cause obesity?

A study of eight countries concludes there's a strong correlation between obesity and the size of supermarkets. The authors suggest large supermarkets emphasise quantity over quality.

National obesity prevalence (BMI ≥ 30 kg/m2) and average supermarket store size (aisle length in metres) in eight countries

Here's an interesting study that finds an extraordinarily strong correlation (r=0.96) between national obesity levels and the size of supermarkets in eight countries with western-style diets i.e. Netherlands, Sweden, Denmark, Canada, England, Australia, New Zealand and the United States.

The paper is titled The correlation between supermarket size and national obesity prevalence and it was written by Adrian Cameron, Wilma Waterlander and Chalida Svastisalee.

The researchers took national-level obesity data from the UK National Obesity Observatory. The size of supermarkets was inferred from the total length of aisle in a sample of 170 supermarkets across the eight countries.

The authors say their finding suggests that "either the large supermarkets themselves, or an aspect of the urban environment or food/shopping culture that encourages such stores, is particularly obesogenic".

They propose a possible narrative:

Explanations for the association between store size and national obesity prevalence may include larger and less frequent shopping trips and greater choice and exposure to foods in countries with larger stores.

Large supermarkets may represent a food system that focuses on quantity ahead of quality and therefore may be an important and novel environmental indicator of a pattern of behaviour that encourages obesity.

I looked at data available at the UK National Obesity Observatory. It shows the age-standardised national average BMIs for the three lowest-scoring countries in the exhibit range from 19.3 to 20.3. Those for the other countries range from 28 to 33.5 (the US, of course, is highest).

That's a huge difference; is it possible it could be explained wholly or in large part by national differences in the average size of supermarkets?