The good news for online retailers around the world and the technology companies that support them is that the global e-commerce market is big, and it’s growing fast. Online retail spending reached $1.74 trillion globally last year, and it’s been growing at roughly 20% per year over the past three years, according to Internet Retailer’s just-released report Global 1000: The global reinvention of e-retail.

The bad news for most e-retailers looking to build major online retail operations is that the global e-commerce market is dominated by a small group of very big players, and their command online retail spending continues to grow. When looking at gross merchandise value (GMV), which is the total value of products sold on websites around the world, the top 10 largest companies—with Alibaba Group Holding Ltd. and Amazon.com Inc. at the top—control nearly half of the global e-commerce market.

Globally, Alibaba holds roughly 27% market share by Internet Retailer’s analysis. Amazon takes 13%. Both companies dominate in their home countries (and to a lesser extent abroad) using a multipronged onslaught of strategies that give consumers a giant selection of products with low prices and fast delivery. They attract other merchants to sell on their marketplaces by providing them access to a boatload of loyal shoppers and convenient options to help them fulfill orders and sell to consumers around the world.

But findings of the Global 1000 reveal that there are some significant market share shifts at the very top. For one, Amazon (No. 1 in the Global 1000) and JD.com in China (No. 2)—both of which sell their own goods and are rapidly expanding their revenue from other merchants selling on their marketplaces—are gaining market share in their home countries at a very fast clip.

Nearly one-third of all online retail transactions in the U.S. occurred on Amazon sites last year, compared with roughly 20% in 2012. Similarly, JD grew its market share in China to 11.3% last year from 5.1% in 2012.

Meanwhile, Alibaba’s share of China’s e-commerce sales declined to 78.5% last year from 81.1% in 2014. That’s despite its impressive sales growth of roughly 43.4% in each of the last few years. While Alibaba remains dominant in China, aggressive competitors like JD are growing more quickly. In the United States, eBay Inc.’s e-retail market share has fallen steadily amid lackluster sales over the past several years.

While the growing domination of the top 10 may be bad news for many online retailers jockeying for their piece of the growing e-retail pie, plenty of others have chosen to partner with the likes of Amazon, Alibaba, JD.com and others by selling on their online marketplaces.

More information is available in the Global 1000 on how the biggest marketplace operators are making it easier for other merchants to grow their businesses online and reach new crops of loyal customers. The report includes, for example, rankings, GMV figures and other key metrics on the 18 largest online marketplaces in the world.

The Global 1000, available in PDF and online database formats, also includes:

• Rankings, profiles and a three-year history of web sales figures on the largest 1,000 online retailers in the world.
• Key statistics on each merchant, including monthly traffic figures, conversion rates and average order values.
• Global and regional e-commerce sales figures for the past five years.
• Web sales, market share, number of online shoppers and other stats about 12 key online retail markets, including the U.S., China, Japan, Germany, United Kingdom, Russia and India.
• Interviews with key e-retail leaders from global e-commerce merchants including Zalando SE (No. 25 in the Global 1000), B2W Digital (No. 26), Debenhams Plc (No. 106) and Amazon.com.

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