You can't compare Walmart to Amazon, Walmart is a northamerican thing, Amazon is global. Walmart has zero influence in the prices of the tomatoes grown in Europe.
Their international scale is similar, though I'm sure not in exactly the same places. Amazon had $118 billion in non-US revenue in the most recent fiscal year while Walmart had $101 billion.
If you limit it to only retail, Walmart probably comes out ahead.
Alas, them deciding to shut-down Book Depository means I can't purchase English/American books anymore free of customs-related taxes (I live in a Eastern European EU county), which makes me not purchasing those books at all (in many cases the taxes would end up doubling the purchase price, so it's not worth it). I'll never forgive Amazon for that.
If you limit it to only retail, Walmart probably comes out ahead.