This goes beyond just groceries. I've lived in a few countries that vary in population from hundreds of millions to just a few million and the impact of scale is very noticeable.

It's very intuitive, but the actual impact is eye opening. I've found that for almost all products and services, the size of the US results in far more companies offering said product/service, which results in a lot more competition and better prices.

You see it not just across consumer items, but things like financial services.