This Fast Company article from December 2003 documents some of the practices of Wal-Mart and its negotiations with suppliers.
Small business often focuses on how a major competitor like Wal-Mart in the US or Woolworths, Coles, Harvey Norman or Bunnings etc in Australia might have on their business. As the Fast Company article outlines, the impact can be felt way down deep in the supply chain. Small businesses are not as tough in negotiations in part because they are not as powerful. Therefore they carry themselves the financial pain inflicted by big competitors on suppliers.
Maybe this is why newsagents get 5% commission for selling Vodafone recharge compared to the 16% paid to a major supermarket chain. Who knows? The greed of some of these giant companies inflicts a pain which is felt not only by suppliers and the sooner the broader community understands that their discounts are not as beneficial as the TV commercials make out.