In the land of "one man, one vote," each ballot is supposed to carry the same weight. But Bill McCollum, the new Republican congressman from Florida's sunny 5th District, has nearly four times as many constituents (nearly 890,000) as Rep. Robert Garcia, the Democrat from the desolated South Bronx (233,787), according to the 1980 census.

U.S. voters were so shifty during the 1970s, geographically speaking, that they have messed up the work done by state legislatures after the 1970 census to create 435 congressional districts all with roughly the same number of people.

Now the state lawmakers have to go through the complex and politically dog-eat-dog task again, so that such population peaks and valleys are leveled to within a percentage point or so of difference, at least within each state.

The new ideal average size of a congressional district, nationwide, is 519,234 people, up from about 466,000 during the 1970s. But because districts do not cross state borders, the actual average is based on each state's population divided by the number of its representatives. South Dakota, which lost one of its two seats, will have one huge district of all its 690,178 residents. Montana, which has barely enough people for two congressional seats, will have two small districts of just over 393,300 each.

Figures from the 1980 census, released today, show how far each district now differs from the average. This is important because "the courts in most cases have required a deviation of less than one percent from the average," noted Don Starsinic of the Census Bureau, whose office prepared the figures.

The biggest deviation is in Florida's 5th, which, after increasing its population by 94 percent, is a whopping 71.7 percent more populous than the state average of 512,631. Florida's 10th and 11th districts are not far behind Garcia's district, on the other hand, after losing half its population since 1970, is short of the state average (516,391) by 54.7 percent.

Twelve of the nation's 435 congressional districts increased by more than 50 percent in the last decade, and 58 grew by more than 30 percent, the census showed. Overall, about three-fourths gained population.

California's 43rd, in the San Diego area, is about 65 percent too populous, and Texas' 7th, in the Houston area, is equally overstuffed.

During the decade, 102 districts lost population, 88 of them in northern metropolitan areas. New York's 12th District, in Brooklyn, is about 38 percent too low.Michigan's 13th, in Detroit's inner city, is more than 43 percent too low.

Districts like Virginia's 10th in surburban Washington, which has almost exactly the right number of people, are rare.

After the 1970 census, new districts were carved to within an average of only one-half percentage point of difference by the 1972 elections, Starsinic said. "But it doesn't take many years for the situation to get out of focus."

This time around, for the first time, computers are being plugged into the process in a big way, primarily by the Republicans, who have invested millions in their effort to wrest control of Congress away from the Democrats.

Seventeen U.S. congressional seats will shift from heavily Democratic power centers in the Frost Belt to the Republican-leaning Sun Belt. The most troublesome work faces legislators in states of declining population, where they have to decide whose seats get eliminated.

Movement within states, generally outward from the cities to less populated areas, has meant that even states that neither lost nor gained U.S. House seats have major surgery to perform on legislative districts for state offices. s

While the Supreme Court, in the "one man, one vote" decisions of the 1960s, has required virtual equality in U.S. congressional districts, it has been more lenient in allowing deviations in state legislative districts, according to a specialist with Common Cause, one of several public interest groups pushing for stricter adherence to the one man, one vote principle.

In an important Virginia case in the early 1970s, the Supreme Court allowed a 16.8 percent difference between the smallest and the largest district, the specialist noted, on the grounds that the state has a legitimate policy of respecting county boundaries in its redistricting.

The tussle is under way in Virginia, one of the few states to have completed some portion of their redistricting. Several interest groups have mounted court challenges to the state's plan, charging that it dilutes the voting strength of blacks and violates the spirit of equal representation.

Maryland, like Virginia, retains the same number of congressionals seats. But, because so many people moved from city to suburbs or to rural areas, it has to make major changes in district boundaries, Starsinic said. For instance, the 6th District, which stretches from the Baltimore suburbs to the foothills of Appalachia, has nearly 20 percent too many people, while the 7th, a city district drawn to include most of Baltimore's blacks, is now more than 20 percent too low in population.

Even Maryland's 1st District, known as the sleepy Eastern Shore, showed an unaccustomed jump in population, so that it is more than 15 percent above the state average.