This is certainly not true of Baltimore. Most "corner" stores sell inferior goods (last time I tried to buy canned cranberry sauce at a neighborhood I had a choice of cans expired by 2 or 3 years (sic!) at a convenience store prices (usually higher than supermarket).
What does make a difference is region. Supermarket food in central PA is substantially lower than in Baltimore, and in addition central PA has Amish markets that sell good quality meat and produce at prices below those found in most supermarkets. But it is a supply-driven phenomenon. PA also has the largest number of family owned farms in the union.
Unlike supermarkets, mega-stores, like Walmart or Home De(s)pot, do not compete with local producers, but with local distributors, and because of their size the mega-stores have lower per-unit transaction costs, which makes them more competitive price-wise than mom-and-pop hardware or clothing stores.
However, their lower per-item cost does not necessarily translate into a lower total cots, as they often impose cost on the entire community e.g. job displacement, tax breaks, etc.
Wojtek