Amazon's current market cap is $466 billion; Walmart's is just $228 billion.
Amazon has fully come into its own in the past few years. The company has firmed up its position as the most dominant force in online retail, and has arguably become the most important player in retail period. Amazon has also finally found a product, the Echo, that showed it can become a fully diversified tech company (its Fire phone did not work out so well). The soaring popularity of its Amazon Prime subscription service and the ubiquity of its cloud services have also lifted its share price.
Since May 15, 1997, when Amazon went public, the company's share price has climbed 39,000 percent, while Walmart is up "just" 406 percent. In other words, a $10,000 investment in Amazon when the company went public in 1997 would now be worth $3.9 million, compared to a little over $50,000 if you'd put the money in Walmart instead back then. Amazon passed Walmart's market cap in 2015, and has since added a whole other Walmart in value in the ensuing years.
In the process, Amazon CEO Jeff Bezos has become the second-richest man in the world, and could soon pass Bill Gates for the No. 1 spot.