You certainly chose a clear difference between Costco & WallyWorld.

Try looking at the profit/employee in the table http://www.thedailybeast.com/articles/2012/11/26/why-can-t-walmart-be-more-like-costco.html. Report your findings here.

What do you notice? Costco has a more highly paid labor force--but that labor force also brings in a lot more money. Costco's labor force, paid $19 an hour, brings in three times as much revenue as a Walmart workforce paid somewhere between 50-60% of that. (There's a bit of messiness to all these calculations, because of course both firms have employees who don't work in stores--but that's the majority of their workforce, so I'm going to assume that the differences come out in the wash.)

This is not because Costco treats its workers better, and therefore gets fantastic productivity out of them, though this is what you would think if you listened to very sincere union activists on NPR. Rather, it's because their business model is inherently higher-productivity. A typical Costco store has around 4,000 SKUs, most of which are stacked on pallets so that you can be your own stockboy. A Walmart has 140,000 SKUs, which have to be tediously sorted, replaced on shelves, reordered, delivered, and so forth. People tend to radically underestimate the costs imposed by complexity, because the management problems do not simply add up; they multiply.