Walmart probably plans the way it grants hours to workers to match welfare subsidies. It knows how many children its workers have, whether they are married, where they live and lots of other data. No Ma & Pa store could successfully use this data to its own advantage.
Recently, Aldi's a German chain, has opened up a lot more stores, and they have WalMart beat on price and convenience, since they do not make customers wait in line so long, and are much smaller, parking problems are fewer. It is easier to find the groceries you want in a smaller store. If you buy house brands, Aldis as well as Save-A-Lot have WalMart beat on prices. Both of these do make you bag your own groceries, and pay for the bags, if you do not bring your own. WalMart makes it hard to bag your own, as they use a bagging carrousel. When Public=x has a sale, their prices are lower than WalMart, and the quality is always the best.
Aldi's owns Trader Joe's, that has the same format as Aldi's, but there are fewer Trader Joe stores.