Amazon is crushing Walmart, eBay, and Target in the growing mobile shopping space

Amazon is already the largest online shopping site in the world, but it's also showing signs of dominating the next most important shopping platform: mobile shopping.

According to a new note published by Oppenheimer this week, Amazon's lead in the mobile shopping space is growing by a wide margin, tripling the number of US unique visitors to its mobile app over the past two years. Meanwhile, its biggest competitors like Walmart, eBay, and Target saw almost no growth in the same time span.

On top of that, Amazon's mobile app is increasing its penetration rate in the US, with 50% of US online shoppers now on its app.

"At the end of 2014, Amazon had roughly the same number of mobile unique visitors as Walmart and eBay, in the US. As of December 2016, Amazon has more unique visitors than the apps of those two companies' combined," Oppenheimer wrote in the note.

"Amazon is well positioned to benefit from the shift to mobile shopping," it added.

These changes are particularly important for Amazon as more online shopping is expected to move from desktop PCs to mobile devices (typically smartphones or tablets) in the near future.

BI Intelligence forecasts mobile commerce sales will reach $284 billion, or 45% of the total US e-commerce market, by 2020 - up from $33 billion, or 11% of the total US e-commerce market in 2014.