The Capital Beltway has become a major dividing line for metropolitan Washington, with suburbs inside it resembling the District of Columbia in small household size and declining population while areas beyond continue to grow, a new analysis of 1980 census data shows.

"The real city, as opposed to the political city, now seems to be bounded by the beltway, not the D.C. line," said George W. Grier, a demographer who prepared the analysis for the Greater Washington Research Center.

"The beltway itself helped transform the areas beyond it . . . . The inner parts of Montgomery, Prince George's and Fairfax, as well as Arlington and Alexandria, now are a great deal like the District of Columbia."

During the 1970s, the new analysis indicates, every jurisdiction or county area inside the beltway declined in population. The total inner-beltway loss was 206,105 people, or 11.6 percent of its 1970 population. Outside the beltway, every jurisdiction grew substantially, for a total population gain of 357,085, or 31.4 percent.

Household size dropped sharply in all parts of the area. By 1980, average household size within the beltway was just 2.43 persons per household, with singles and childless couples more numerous than large families.

Outside the beltway, the average household had 3.02 persons in 1980, indicating that families with children still predominate, the new report says.

Atlee Shidler, executive vice president of the nonprofit research center, suggested that the dispersal of population occurred, despite rising fuel costs and the opening of the Metro subway, largely because almost all the increase in jobs in the area since 1970 took place in the suburbs.

Also, housing costs remained lower outside the beltway, Shidler said, attracting families who want space for their children.

"It all goes contrary to the planners' wishful thinking," Shidler said. "But the suburbs now are producing their own suburbs. Virtually all of the employment increase has been outside the city, a lot of it along the beltway at Tysons Corner or around Montgomery Mall, also out Rte. 270 or I-95. For people who work in these places, it's a lot easier and cheaper to live in upper Montgomery or outer Fairfax or even Frederick County or Prince William than to stay inside the beltway. It probably uses less energy, too."

Even though cost of gasoline soared after 1973, Shidler said, "it came nowhere near to balancing" the cheaper land and housing costs in the outer-beltway area.

The new Metro may encourage new offices downtown, he said, but at the same time it makes it easier for people to live in the outer-beltway suburbs.

"Already a lot of people are parking at New Carrollton at the end of the line and taking Metro to their jobs," Shidler said. "The subway doesn't necessarily encourage residential concentration."

The new report was written for the center by George and Eunice Grier, partners in a demographic consulting firm in Bethesda. The Griers used to work at the Washington Center for Metropolitan Studies, which became part of the research center in 1978.

Yesterday George Grier noted that when the 65-mile beltway was completed in 1964 it encircled virtually all the developed parts of the metropolitan area.

"Outside the beltway we have a more recent period of development," Grier said. "Inside we have an earlier period with houses that are aging, children that are leaving home, singles and childless couples who are finding opportunity. It's a natural development, not some pathology. It isn't the District line that has much significance."

Despite the population loss in all inner-beltway jurisdictions, Grier noted that the area had a 3.2 percent rise in households from 1970 to 1980, reflecting modest new construction. In the District itself, though, there was a 3.2 percent decrease in households. Alexandria and inner-beltway Fairfax had household increases of 15.4 and 26.2 percent respectively, though apparently, Grier said, most of these new households were made up of singles or very small families.

Outside the beltway, Fairfax County virtually matched the high growth rates of Prince William, Loudoun, and Charles counties, and exceeded all three combined in the number of new residents. Upper Montgomery County grew by a substantial 21.4 percent in population and 49.1 percent in households, even though Montgomery's inner-beltway area lost 11.6 percent of its population.

Prince George's County had the smallest population growth outside the beltway, just 15 percent since 1970. Grier noted that the slowdown there occurred after the start of a major busing plan in 1973, which was followed by a massive loss of white students from public schools.

Overall, 49 percent of metropolitan Washington residents lived outside the beltway in 1980 compared with 39 percent 10 years earlier.