In a famous episode of “Seinfeld,” Jerry complains that people constantly assume he's gay because he’s single, obsessively neat — and thin. As it turns out, at least part of that punchline may be anchored in fact.

A new study shows that gay men really are leaner than straight men. And conversely, it also found that gay women tend to be heavier than their heterosexual counterparts.

Boston researchers determined that gay women were more than twice as likely as straight women to be obese, while gay men were 50 percent less likely to be obese compared to their heterosexual counterparts, according to a report published in the American Journal of Public Health.

After scrutinizing a health survey of more than 67,000 Massachusetts residents between the ages of 18 and 64, the researchers found that 14 percent of gay men were obese versus 21 percent of straight men. The opposite was true of gay women: 26 percent were found to be obese, as compared with 17 percent of the straight women.

The researchers also found that both gay men and gay women were more likely to be current smokers compared to their heterosexual counterparts. And gay women were more likely to have multiple heart disease risk factors than straight women.

Earlier studies that looked at health in the gay community focused mostly on sexually transmitted diseases and mental health, rather than on chronic illnesses like obesity and heart disease, says the study’s lead author Kerith Conron, an associate research scientist at Northeastern University and a research fellow at the Harvard School of Public Health. “This may mean that we in the public health community need to come up with more tailored messages to reach these groups, just as car dealers do when they want to reach a specific target audience,” she said.

Conron suspects that cultural differences might at least partly explain the weight divide. It may be more acceptable in the lesbian community for women to be full-sized, she said.

That explanation makes sense to Esther Rothblum, a professor of women’s studies at San Diego State University. “People in sexual relationships with men — heterosexual women and gay men — get more pressure to look thin and to otherwise conform to attractiveness norms than do people in sexual relationships with women — lesbians and heterosexual men,” Rothblum said.

The best evidence for that comes from an older study of bisexual women, Rothblum adds. When the bisexual women were asked to describe their experiences with women and men, the differences were dramatic: they often reported that they got more pressure to be thin when they were with men.

Other researchers have found further evidence of this male effect while studying personal ads and dating sites like Match.com, Rothblum explains. “Men will say they are looking for a partner who is not above 35 years old and not above 135 pounds,” she said. “Women don’t typically do that. They say they are looking for someone with good sense of humor, intelligence, and creativity, or someone who is not an alcoholic.”

Linda Carroll is a health and science writer living in New Jersey. Her work has appeared in The New York Times, Newsday, Health magazine and SmartMoney.