A recent post on Motley Fool highlighted that most analysts have yet to realize the actual amount of sales Amazon drives:

Remember, while Amazon dominates e-commerce, its sales pale in comparison to brick-and-mortar operations like Wal-Mart (NYSE:WMT) which logged more than four times the revenue that Amazon did last year. Even Costco Wholesale (NASDAQ:COST) tallied more annual revenue than Amazon, yet the e-commerce giant managed to outgrow all its competitors in the U.S. combined. For an industry with such low barriers to entry, that is not an easy feat.

Comparing Wal-Mart sales to Amazon sales is apples to oranges. Amazon has two typses of sales: 1) first-party - sales of products they purchase and inventory themselves; 2) third-party - products sold and/or fulfilled via Amazon properties. Amazon only reports the first-party sales and the commissions earned on third-party sales. The third-party commissions range from 10-15% of the actual sale price on-site. Therefore, Amazon is reporting a number well below the actual sales driven on their various platforms.

If I was to estimate the actual sales number, it would be closer to $200-$250 billion annually. That would mean Wal-Mart is about two times the size of Amazon in sales. Anything above $116 billion (Costco) would make Amazon the 2nd largest retailer in the world.