The definition of "big" or "large" is subjective to the person who is measuring the activity.
For example, Google measures its own metrics on the amount of online traffic that goes to its servers and how many clicks are happening.
Google also measures the amount of text searches that happen. They publish these numbers every month in their quarterly earnings reports. These are all examples of Big E-Commerce measurements.
Amazon does not have this type of data for any other country in the world. So, it is a big e-commerce metric that no one else will be able to provide.
Big E-Commerce is a highly watched metric in the United States because it has so much to do with the economy in general, specifically U.S. jobs, &
Also because investors want to make sure they are getting their fair share of each online retailer's activity (i.e. selling their products online).