's extraordinary growth has turned Seattle into the biggest company town in America.

Amazon now occupies 19 percent of all prime office space in the city, the most for any employer in a major U.S. city, according to an analysis conducted for the Seattle Times.

The e-commerce giant's presence in Seattle is more than twice as large as any other company in any other big U.S. city, the analysis showed, and its expansion is just getting started.

Swarms of young workers crowding into South Lake Union every morning represent an urban campus unparalleled in the United States - and they have helped transform Seattle, for better or worse. Amazon's rapid rise has fueled an economy that has driven up wages and lowered unemployment, but also produced clogged traffic on the roads and sky-high housing prices.

And while Seattle's booming economy is often attributed to a wide variety of factors, increasingly, it's all about one: Amazon, which now occupies more office space than the next 40 biggest employers in the city combined.

Its 8.1 million square feet is expected to soar to more than 12 million square feet within five years.

Amazon's supremacy in e-commerce and cloud computing has translated into an avalanche of glass, steel, people and money. Its unparalleled impact may give pause to those who experienced the downturn of the 1970s, when the shine of "Jet City" was tarnished as Boeing cut about two-thirds of its huge local workforce.

"Seattle's been through this before," said Tracey Seslen, a professor at the University of Washington's Foster School of Business. "If Amazon were to leave, that would create a giant hole in their wake."

Unlike Boeing, however, Amazon runs a vast web of mutually reinforcing yet diverse businesses - selling computing power, retailing nearly everything, publishing books and producing films, among other things.

John Schoettler, Amazon's director of real estate, said all he's experienced in nearly two decades is "steady, continued growth," the result of the company's zealous focus on satisfying customers.

The legacy of what so far amounts to $4 billion spent by the company on real estate in Seattle will be long-lasting, he said: "These buildings will stand for hundreds of years."

For this story, the real-estate data firm CoStar provided a list of all office tenants in the nation's 20 biggest cities by population, looking at only Class A offices, the modern buildings used by the vast majority of major employers.

Though other company campuses may be larger and more dominant in some suburbs - Microsoft in Redmond, or Apple, Google and Facebook in Silicon Valley - in big cities, corporate tenancy is generally fragmented.

For example, financial giant Citi has 3.7 million square feet in New York - making it the second-largest major-city office tenant in absolute terms after Amazon. But it represents less than 3 percent of Class A office space in the Big Apple.

That's typical: In most big cities, the top employer has less than 5 percent of local office space.

Among the largest 20 U.S. cities, only Columbus, Ohio - where insurer Nationwide occupies 16 percent of office space - has a situation comparable in its dominance to Amazon. But it's still less than half of Amazon's total square footage.

In Seattle itself, Amazon is in a league of its own. Its presence is nearly 20 times greater than that of the next-biggest employer.

Next in size after Amazon, Safeco Insurance and the University of Washington each occupy about 400,000 square feet of office space in Seattle, followed by several companies with about 300,000 square feet each: Tableau; Zillow; Zulily; Nordstrom; F5 Networks; Facebook; and the U.S. Department of Labor.

Amazon got its start in a Bellevue garage in 1994, and it first grew without much of a plan - its employees were scattered in various downtown Seattle buildings and in the former Pacific Medical Center building on Beacon Hill. When Schoettler, the Amazon real-estate executive, joined the company in 2001, it had 630,000 square feet in Seattle.

In 2005, Schoettler said, he told CEO Jeff Bezos that the company needed a plan, and Bezos agreed. His only condition was that Amazon stay in downtown Seattle, Schoettler said.

That coincided with the reversal of a decades-long outflow from U.S. cities to their suburbs: By staying in the urban core, Amazon would attract members of the hip creative class.

"It was a very conscious decision we made," Schoettler said.

Under Amazon's current plans, there will be room in Seattle for more than 55,000 Amazonians at the beginning of next decade.