The population of Prince George's County is now the largest in the Washington area, surpassing the District of Columbia by more than 22,000 even though both jurisdictions lost people during the 1970s, the U.S. Census Bureau reported yesterday.

According to the bureau's official 1980 count, Prince George's has 657,707 residents, down about 4,000 from 1970. Washington's population, reported last month, was 635,185, down more than 121,000 in a decade.

Overall, the census bureau said, the population of the Washington metropolitan area grew by just 4.5 percent from 1970 to 1980, reaching a total of 3,041,812. This modest increase, most of it in Northern Virginia, contrasts sharply with the area's 38 perent population increase in the booming 1960s.

Despite the slight population gain, the number of housing units in the area climbed by a substantial 24 percent, indicating a major decrease in average household size because of fewer births, more divorces, and more young singles and elderly living alone, a pattern that has occurred nationwide. Even so, the percentage increase in housing units in the past decade was just half what it had been during the 1960s.

The only parts of the area whose population grew rapidly during the 1970s were outside the Capital Beltway. Fairfax County had a 31 percent population increase to 595,576, topping Montgomery County for the first time by 21,000.

Farther out, both Loudoun and Prince William counties in Virginia grew by 54 percent while the population of Charles County in Maryland rose by almost 52 percent.

The new figures, which give the population as of April 1, are higher than earlier incomplete data which was presented to local officials for review in mid-summer. The census bureau said the numbers are still considered preliminary because minor errors in arithmetic may yet be found, but they said any further changes are expected to be very slight.

Officials in Fairfax County have speculated that because of rapid growth their county will soon become the largest in the Washington area. But census officials cautioned against predicting the future directly on past trends.

Prince George's, for example, was the fastest-growing part of the area during the 1960s, with an 85-percent inrease that also made it the second-fastest growing large county in the United States after Orange County in southern California.

There was a real turnaround this time in Prince George's," one census official said. They managed to hang on there better than the District, but they really turned off the growth."

Aside from the changes in household composition that tended to decrease population everywhere, Prince George's also was affected by a sewer moratorium that halted new construction in the early 1970s and by a massive court-ordered busing program for school desegregation. The busing went into effect in January 1973 and was accompanied by a sharp drop in whites school enrollment.

The new population data contain no breakdown by race, but earlier estimates indicated a major decline in the number of whites in Prince George's, which was almost offset by an increase in blacks.

In contrast to the 1960s, when developers covered much of the county with relatively low-rent apartments, Prince George's changed its development policy after 1970 to discourage low-cost housing. Instead, the county government, led by former executive Winfield Kelly, sought to attract a "new quality" of development of expensive single-family homes. The policy met with little success.

"Construction in Prince George's never came back like it did in Montgomery," said census bureau analyst Joel Miller. "They cut off their natural market [low- and moderate-priced housing], and there wasn't much else to replace it."

Miller said population dropped by 9 to 16 percent in the older parts of Prince George's inside the Beltway, such as College Park and Hyattsville. pAlthough there was growth elsewhere, particularly around Laurel, Camp Springs, and Tantallon, it was not enough to offset the decline. In Bowie, there was a 4 percent population loss, as many of the familes who first moved into new houses in the late 1950s and 1960s grew older and their children left home.

"The same thing is happening in many older suburbs around the country," Miller said.

The 16 percent population loss in Washington is similar to what took place in the 1970s in many other older big cities. The census bureau reported a 13.5 percent drop in Baltimore and a 12 perent decline in Richmond.

In Arlington, which is the oldest and most densely developed suburb in the Washington area, the population dropped by 12.9 percent from 1970 to 1980, falling to 151,885. Alexandria had a 7.6 percent loss, the census bureau said, to 102,494, despite substantial new construction.

"Most of the new housing was for singles and young couples," Miller said. It's condo land, and usually when something is converted [from rental to condominum] the population falls because the families move away."

According to the new census reports, major growth during the 1970s occurred just outside the metropolitan area in Howard, Calvert, and Frederick counties in Maryland and in Stafford and Spotsylvania counties in Virginia. c

These areas apparently attracted families with children because of relatively low housing prices, Miller said, despite the sharply higher cost of gasoline for driving to work.

In Howard, principally because of the new town of Columbia, the population rose by 89.8 percent during the 1970s to 118,443 this year. Spotsylvania County, near Fredericksburg, Va., grew even faster, by 109 percent, but its population total reached only 34,336.

Because of the population shifts, the District of Columbia now accounts for just 21 percent of the population of the metropolitan area, compared to 26 percent in 1970 and 36 percent a decade earlier. Ten years ago the District had 95,000 more people than Prince George's. In 1960 it had 406,000 more.