Walmart screws the farmer with its Monopsony-power and uses that to undercut the small operater with its Monopoly power.
However much I might be inclined to agree with you, WalMart clearly just exploits the differences between two of the largest and least competent governments on Earth, via the loophole of global free trade. The Western farmer is 'screwed' because WalMart has opened a market of a billion consumers, while the Western retailer is screwed because WalMart can exploit those billion consumers as manufacturers. But they can only do this because both governments have abrogated basic responsibilities and instead of preventing abuse and trade imbalance, sanction it.