> While the package on the shelf might seem the same price as in a more mainstream supermarket, it in fact holds less contents.

They are often just the same packages at higher prices, the more “upscale” supermarkets can make up for the lower margin on goods both stores sell with higher margin goods that wealthier clientele will buy alongside core goods (things like in-store cut deli meats and cheeses, high-end alcoholic beverages, gourmet cheeses, in-store kitchen prepared ready-to-eat food, etc. When both kinds of stores exist within a radius where they compete with customers—the wealthier being more mobile—they use this fact in marketing, too, because this also let's them win same-good price comparisons.