If you thought that e-commerce sales would plateau after a busy holiday season, think again.

According to eMarketer, annual global e-commerce sales are set to double over the next four years, to about $4.1 trillion. In 2016, all e-commerce sales worldwide hit $1.9 trillion, up from $1.5 trillion in 2015.

And the world’s top two online retailers are barely tapping into a fast-growing trend, even though their sales keep increasing at a rapid clip.

For example, Amazon’s revenue climbed 22 percent year-over-year. Additionally, the company’s total 2016 sales increase 27 percent, reaching a whopping $136 billion.

Walmart showed healthy growth in its e-commerce business last year, increasing more than 15 percent year-over-year, and its U.S. e-commerce sales gained 35 percent on the prior year’s quarter.

It highlighted several recent e-commerce and omnichannel investments that helped drive its online sales growth such as:

• Acquiring e-commerce Jet.com, which contributed heavily to its U.S. e-commerce growth
• Orders via the Pickup Today service, which enables same-day pickup for items ordered online and makes them available in domestic stores
• Launching a Global Import Store on JD.com, the China-based e-commerce giant in which it owns a 12 percent stake

Warding Off Competition

To keep Amazon in its sights, Walmart continues to enhance its online presence. At the same time, it is exploring ways to bridge its online and in-store sales and operations. Recently, they began offering free two-day shipping on online orders of more than $35. This esulted in an uptick in online sales.

According to Business Insider, this shipping policy is clearly aimed at undercutting Amazon, which offers free two-day shipping for its Prime members who pay an annual $99 membership fee. Walmart also acquired online shoe retailer ShoeBuy and outdoor apparel and gear e-tailer Moosejaw, in efforts to gain more online sales, while expanding its product offerings for a higher-end customer segment.

A Long Road Ahead

Even with its impressive growth and investments, Walmart’s total global e-commerce sales still make up a small fraction of its business. For example, online sales make up only 3 percent of total sales. This means that the retailer lags behind Amazon with a long way to go in the global e-commerce space.

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