Heading into the holiday shopping crush, there's really no comparison between Amazon and Wal-Mart when it comes to vying for shopper dollars on the Internet, said Gian Fulgoni, co-founder and chairman emeritus of digital analytics firm comScore.
Amazon has 20 percent market share of all online spending, Fulgoni told CNBC's "Squawk Box " on Wednesday. "Amazon's share is higher than the next six retailers combined. That's how powerful they are. And there's no evidence that things are slowing down for them."
He said Wal-Mart, for all its dominance in the brick-and-mortar world, has just 4 percent online market share, followed closely by Target. "Everybody now realizes the importance of the online channel. You just can't ignore it. You do so at your own peril."
Second to Amazon in the pure online retailers is eBay at about 10 percent, Fulgoni said.
ComScore predicts a 14 percent rise in total digital commerce this holiday season to $70 billion. Fulgoni said the online spending growth surges to nearly 50 percent on mobile devices to account for $11.7 billion. "There's a channel shift from buying in store to online, and then kind of a secondary shift, which is people increasing buying with mobile devices."
"There is a limit though, I think, to how much will be spent on the phone because the size of the screen," he said, adding that big ticket purchases that require lots of research favor desktop computers.
Online purchases from desktop is expected to grow about 9 percent, Fulgoni said.
ComScore expects Cyber Monday, the first Monday after the Thanksgiving, to surpass $3 billion, becoming the heaviest online spending day in history for the sixth-straight year, "with roughly half a billion of those dollars coming from mobile devices."