That aside, I'm sorry but you're just wrong about this. The data is pretty unequivocal: larger firms have more scope for investments in process, technology, and tools that lets them operate more efficiently. In the example of Wal-Mart, they run an ultra-efficient supply chain that minimizes breakage, makes sure popular goods are available for purchase, etc.
Whereas half the time when I go into a non-chain grocery, they're out of, or otherwise not stocking the thing I want, they're closed, the item isn't in the right place on the shelf, it's hidden in the "back room", a supplier is late, or there's some other problem. Sure, some of this is due to bargaining power, but Wal-Mart also operates at a scale where they can afford to invest billions in sophisticated merchandising systems. As software people, we should understand what a difference this stuff makes to business.
Small firms pay worse because they simply don't make as much, given the same level of sales, as large firms do. The argument is a complete strawman. Lower, less-efficient firms don't have the gross profits (price - cost of goods sold) to be able to afford higher wages, even if they wanted to.