Fatty tissue has been found in the lungs of overweight and obese people for the first time.
Australian researchers analysed lung samples from 52 people and found the amount of fat increased in line with body mass index.
They said their findings could explain why being overweight or obese increased asthma risk.
Lung experts said it would be interesting to see if the effect could be reversed by weight loss.
In the study, published in the European Respiratory Journal, scientists looked at post-mortem samples of lung donated for research.
Fifteen had had no reported asthma, 21 had asthma but died of other causes and 16 died of the condition.
The scientists used dyes to carry out detailed analyses of almost 1,400 airways from the lung samples under the microscope.
The researchers found adipose (fatty) tissue in the walls of airways, with more present in people with a higher body mass index,
And they say the increase in fat appears to alter the normal structure of the airways and cause inflammation in the lungs – which could explain the increased risk of asthma in overweight or obese people.
Dr Peter Noble, an associate professor at the University of Western Australia, in Perth who worked on the study, said: "Being overweight or obese has already been linked to having asthma or having worse asthma symptoms.
"Researchers have suggested that the link might be explained by the direct pressure of excess weight on the lungs or by a general increase in inflammation created by excess weight."
But, he said, their study suggested "another mechanism is also at play".
"We've found that excess fat accumulates in the airway walls, where it takes up space and seems to increase inflammation within the lungs," Dr Noble said.
"We think this is causing a thickening of the airways that limits the flow of air in and out of the lungs and that could at least partly explain an increase in asthma symptoms."
Prof Thierry Troosters, president of the European Respiratory Society, said: "This is an important finding on the relationship between body weight and respiratory disease because it shows how being overweight or obese might be making symptoms worse for people with asthma.
"This goes beyond the simple observation that patients with obesity need to breathe more with activity and exercise.