(FORTUNE Magazine) – WHAT WILL CHECK WAL-MART'S GALLOPING GROWTH?

I'm not suggesting it ought to be checked. Despite escalating attacks on the company by labor unions and small-is- beautiful crusaders, I can't help noticing that hundreds of millions of customers--many of them union members and small-town residents--seem to think those stores are just wonderful. But I'm suggesting the growth of the world's largest and most important company will be checked one way or another, and the only question is how. As we approach Feb. 17, when Wal-Mart will announce the staggering total of last year's sales--analysts estimate about $287 billion--consider the following:

Extrapolating any company's early growth is misleading because of the small base. So to get a realistic sense of Wal-Mart's stunning expansion, which many people still don't grasp, look at its results only after it became a true behemoth 15 years ago, when it passed Sears to become America's largest retailer. Back then people were already marveling at the company's stupendous size. It was a giant of the type that usually lumbers rather than sprints. Yet if Wal-Mart were to continue growing for the next 15 years at the same rate it has grown since that time, its revenues would be $3.2 trillion. For comparisons one must look to nations rather than companies. That figure is just shy of Japan's total economic output, or about the size of France's and Britain's economies combined, or of Portugal's, Spain's, Italy's, Greece's, and Turkey's combined.

If Wal-Mart's workforce were to grow for the next 15 years at the same rate as the past 15, the company would employ 8.2 million people. Again, comparisons with other companies are futile, but imagine traveling to Los Angeles and San Francisco, then to St. Louis and Cleveland, down to Miami, up to Atlanta, then on to Philadelphia and Boston, and every person you met, everyone in all those cities, worked for Wal-Mart. That's how many people it is.

If the floor space of Wal-Mart's stores increased for the next 15 years at the same rate as the past 15, it would total 4.2 billion square feet. That figure is incomprehensible, and even expressed in acres (96,419) it's unwieldy, but it works out to 150 square miles--and we're not talking about Wal-Mart's vast parking lots or distribution centers, just stores. You think Las Vegas is sprawling? The interiors of Wal-Mart's stores would occupy more space than the entire land area of America's fastest-growing city.

I don't think any of those things will happen, based on Stein's law. As the economist Herb Stein famously observed, if something cannot go on forever, it will stop. What matters to consumers, employees, suppliers, competitors, and communities is exactly when and how Wal-Mart's growth rate will decelerate. Three forces look almost certain to do the job.

● Societal resistance. We Americans have no problem with extremely big companies, yet we've always set limits. We've often expressed them through federal antitrust action, which in Wal-Mart's case seems unlikely anytime soon; as the company constantly reminds us, it still conducts less than 10% of U.S. retailing. But its recent PR campaign of full-page ads in 100 newspapers ("Wal-Mart is working for everyone"), which the company acknowledges is unprecedented, shows it's feeling heat. Maybe labor unions will get a foot in the door. Maybe Congress will enact some kind of small-business protection. As Wal-Mart expands into ever more lines of business, maybe consumers will limit the dollars they're willing to hand to one company. Which brings up the next factor....

● Competition. The conventional view is that Wal-Mart steamrolls everything in its path. But businesspeople are amazingly creative when desperate, and some of them shocked Wal-Mart this past Thanksgiving weekend when they stocked, priced, and promoted merchandise better than the Beast of Bentonville. Somewhere out there are competitors who will show the world how to survive and thrive against it.

● Cultural exhaustion. The most insidious force of all, it's what finally slowed down mammoths of past decades--General Motors, IBM, AT&T, Sears. The driving spirit fades, risk taking becomes a career killer, process replaces execution. I see no sign of it yet at Wal-Mart, which is remarkable, but to avoid it in coming years will require a near miracle.

CEO Lee Scott is well aware of these dangers. He's continually insisting that Wal-Mart can indeed keep growing. He's absolutely right--it can. But it cannot keep growing as fast as it has been.

GEOFFREY COLVIN, senior editor at large of FORTUNE, can be reached at gcolvin@fortunemail.com. Watch him on Wall $treet Week With FORTUNE, weekends on PBS.