When the Boston Red Sox announced a long-awaited and highly controversial plan to replace venerable Fenway Park, the team made much of the deficiencies of the old ballpark. At the top of the list, however, were not the antiquated restrooms and snack bars or even the rats that occasionally menaced television cameramen. The most criticized feature of the old stadium was the narrow seats.

"In our surveys, seating was the largest single issue for fans who come to more than three games a year," said Kathryn St. John, a spokeswoman for the team. In the new stadium, fans will enjoy an 18 or 19-inch seat instead of a 14 or 15-inch one.

The Red Sox experience was a sign of how obsessed America has become with widening its seats. That's mainly because our own seats have widened. Statistics indicate that the number of obese Americans has increased by 50 percent in the last two decades, and since 1985, the average weight has increased by 10 pounds. Much of this surplus has migrated to the bottom line, and seats in airliners, sport utility vehicles, movie theaters and elsewhere are feeling the impact.

Another ballpark, Tiger Stadium in Detroit, is due to be replaced next year by Comerica Park, now under construction. Its new molded-plastic seats, echoing the shape of the old wooden slatted ones, will be two inches wider than the originals. Many of the new crop of stadiums offer such seats, along with a premium class of wider ones, called club seating, which are usually two inches wider than standard.

"Sure, Americans have always liked things big," said George Kordaris, the editor and publisher of Office Insight, an office furnishings industry newsletter based in Darien, Conn. "But there's no question that American butts have gotten bigger in the last few years. I've seen a lot of chairs from European companies that Americans are just not comfortable in."

Designers and administrators of stadiums and theaters are finding that the standard 18 inches of bench or bleacher space that guides like the Architectural Graphics Standards of the American Institute of Architects recommend no longer suffice for the same number of people.

At General Cinema Clifton Commons, a 16-theater complex in Clifton, N.J., which opened last spring, the seats are wider than those in older theaters, and cup holders are on the seat back in front for elbow room. "One of the positive feedback factors we get is the improved seating," Ron Lichtenberger, the senior manager, reported.

In the air, seats are a key battleground in the design of future airliners, with Airbus bragging that its planned A3XX will offer wider seats than the Boeing 747-400. Boeing, for its part, pushes the virtues of the new 777, whose coach seats are wider that the 747's (18.5 vs. 17 inches).

On the road, sport utility vehicles are growing larger, and so are their seats. When Ford rolls out its new Excursion, 7 inches longer that Chevrolet's Suburban, the Suburban counters with wider seats in its year 2000 models. In Ford's new subcompact Focus, there was no room to widen the seats, but, said Mike Bradley, the designer in charge of the car's ergonomics, Ford did away with constricting lumbar bolsters, flattening the seat bottom more comfortably to accommodate human seats.

In the New York City subway, the new R-110A cars coming to the IRT lines do away with the old scoop seats in favor of a flat bench, which eliminates overlap.

With a certain militancy emerging on the part of larger members of the population, many companies are reluctant to refer outright to larger rears. But the renewed popularity of huge, old-fashioned club chairs, like one in the fall Pottery Barn catalog, for instance, suggests a subtle way to minister to those who want more seat room. So, perhaps, does the return to fashion of such 1950's designs as Arne Jacobsen's broad-armed Swan chair.

What most designers have already realized is that however much the average rear may have expanded, the important news is that it has become more varied. The population has become somewhat taller and a good deal heavier but also far more diverse in size and shape.

This was one finding of the extensive research that went into the design of Herman Miller's Aeron chair, a favorite in high-tech offices. For this reason, William Stumph, Don Chadwick and the other designers decided to produce three sizes of the Aeron. The designers also found that men and women sit differently: Larger men, for instance, lean back to avoid excess weight on their ischial tuberosities, the bony points of the posterior.

Fitting seats to a variety of dimensions is hardly a new goal. Along with all the ergonomic research, the Aeron team also pursued a line of literary research. In Shakespeare, they found evidence of the search for ideal-size seating. The clown in All's Well that Ends Well refers to the search for an imagined chair "that fits all buttocks: the pin-buttock, the quatch-buttock, the brawn-buttock, or any buttock." Such a chair is more elusive than ever.