Men classified as obese earn around $140 more a week than those with a normal BMI, and women who were classified as obese earn $60 less than women with a normal BMI, according to a controversial study.

The Otago University study looked at 1000 Christchurch born 30-35 year olds.

Associate Professor John Horwood said the results were surprising.

"One would expect over time there would be changing patterns of obesity in the sample, and that some of these trends might potentially get worse, some of them might go away."

"It would certainly be of interest to have a look at this cohort again."

However, a recruitment expert is sceptical.

Hays Managing Director Jason Walker said size was a factor hundreds of years ago as the richer could afford to eat more. "It used to be a sign of those things, but we see absolutely no correlation in the workforce."

"It's pretty much based around your ability, experience and qualifications, not size."

He hit out at the methodology of the study, saying "they've actually taken this information and skewed it to make it look as if heavier men earn more income, and lighter women earn more income. There are a lot of variables that they need to add into the mix." Walker added that BMI isn't a great calculation, as it doesn't distinguish muscle and fat.

In contrast, a women's rights advocate aren't surprised at the research.

National Council of Women CEO Sue McCabe said there's more stigma attached to weight for women.

"In a nutshell, our society says that men need to be big, and women need to be small. That's a message we're bombarded with on a daily basis."

"We'd love to see those results change and we can change them. It's as simple as stopping putting pressure on people to conform to a stereotype solely based on their gender."