Different sources give different rankings, often very different. I picked that one because it was the only one I found at the time that put Whole Foods at #6, so at least might be the ordering the article author was thinking of.

It's also not clear how largeness should be measured. Some lists seem to be by sales. Some seem to be by number of stores. Some seem to be by number of employees. It's not always clear which a given list is using.