Just as children often grow bigger and sturdier than their parents, many American suburbs -- including those of Washington -- are now more populous than the central cities that provide their metropolitan identities.

Given recent trends, it was just a matter of time before Fairfax County, with a land mass the size of Los Angeles, would surpass the District of Columbia in population.

According to official Census Bureau estimates reported in yesterday's paper, Fairfax in 1985 became No. 1 in the region with 687,800 people, followed closely by Prince George's County with 678,100 and Montgomery County with 643,400. The District was in fourth place with 626,000 -- slightly less than its prewar population in 1940.

Wow! What a turn in 55 years! In 1930, the District's population of 486,869 plus the total 134,190 population of the Maryland and Virginia suburbs added up to 621,059, less than the 1985 population of any of the Big Four jurisdictions.

In 1930, when Herbert Hoover occupied the White House and Franklin D. Roosevelt's New Deal had not yet attracted an influx of federal bureaucrats, Prince George's was the biggest suburb with 60,095 residents, followed by Montgomery with 49,206. Virginia jurisdictions brought up the rear: Arlington with 26,615, Fairfax with 25,264 and Alexandria with 24,149.

Go back further: In 1870, after the Civil War, Fairfax had 13,000 people. By 1900 its population reached 18,580. Its 1930 population was 36 percent greater at 25,264.

Proportionally, the big jump in the region was in the World War II and postwar boom decade of the 1940s. By 1950, the regional population had reached 2 million with the District making up slightly less than half the total. But even then the District led with 802,178.

It wasn't until the 1980 census, reflecting the building of vast suburban housing tracts served by a freeway network whose expansion was already stalled, that the District dropped to only No. 2 even though its population had declined from its 1950 peak. It took just five years after that for the city to be nudged to No. 4. Too Many Permits

James Chatfield of Chevy Chase wonders what kind of brilliance was involved on Tuesday when Montgomery County detoured traffic away from a construction zone at Elm Street and Wisconsin Avenue right onto the threshhold of another construction zone at Elm Street and Arlington Road near an ordinarily already congested Giant supermarket. "Somebody gave out one too many permits to dig, as far as I can figure out," he observed. The Good Old Days

Recently a letter I sent to a reader in Rockville took three days to arrive there. Edward J. Wojtas of Springfield suggests that in some ways the old days were better.

"The other day at a stamp show," Wojtas wrote, enclosing a photocopy, "I bought a post card . . . mailed in Philadelphia at 11:30 a.m. . . . received at Chambersburg, Pa., some 150 railroad miles west, on the same day, Aug. 17, 1906 at 5:30 p.m. And all for the magnificent price of 1 cent."