Amazon.com boxes are shown stacked near a Boeing 767 Amazon "Prime Air" cargo plane on display in a Boeing hangar in Seattle.

A closer look at Amazon's delivery network illustrates why the company is now ready to make one-day shipping the default for its Prime members.

Amazon is already capable of offering same-day and next-day delivery to 72% of the total U.S. population, including almost all of the households (95% or more) in 16 of the wealthiest and most populated states and Washington, D.C., according to a report published in March by RBC Capital Markets.

The vast delivery network is the result of significant investments over the past four years, a period during which Amazon built out fulfillment centers across the country, nearly tripling its U.S. logistics infrastructure, RBC said. Amazon has added roughly double the amount of distribution space Home Depot currently owns.

That means the company has a huge head start in fulfilling its plan laid out in its latest earnings report to shorten the current two-day free shipping plan by one day for Prime members, who pay $119 a year for fast delivery as well as services like unlimited music, access to the video catalog and exclusive deals.

"We see Amazon's 1-day Prime shipping raising consumer expectations and increasing the cost to compete in e-commerce," Morgan Stanley analysts wrote in a note after the announcement.

The map below from RBC shows the four-year change in Amazon's distribution footprint. It already covers most of the coastal cities, as well as Texas and major metropolitan areas in the Midwest.

"While store-level distribution is still the fastest way for a consumer to acquire a product, Amazon's continued rollout of same-day and next-day delivery capabilities continues to reduce that historical competitive barrier and represents a growing risk to retailers who are too often fighting yesterday's (2-day) delivery wars," RBC Capital wrote in the note.

Following the Amazon announcement, RBC said in another report that "the faster you ship, the more people buy."