Discrimination against the overweight and obese even extends to their partners or friends, researchers have found.
This image made women judge the man negatively
People shown a picture of the same man with a thin or an obese woman rated him more negatively if he was with the fatter woman.
The team from the University of Liverpool say their findings show just how extensive discrimination is.
In Britain, one in four women and one in five men are classed as obese.
Researchers showed 144 women one of the images. They were asked to assess the characteristics of the men - and ignore the women.
People judged the man more positively when he was with the thinner woman
After seeing the picture briefly, they were then asked to rate the men using a list of 50 different characteristics, such as happy or miserable, strong or weak, confident or reserved.
Those women shown the image of the man with the obese woman were more likely to class him as miserable, self-indulgent, passive, shapeless, likes food, depressed, weak, unattractive, insignificant and insecure.
Overall, the men were given a 22% worse rating.
In contrast, women shown the picture of the same man with the thinner woman were more likely to say he was happy and self-confident and less likely to be depressed.
Dr Jason Halford, associate director of the Kissileff laboratory for the Study of Human Ingestive Behaviour at Liverpool University, led the research.
He told BBC News Online it had been known for a long time that obese and overweight people themselves experienced discrimination and stigma - but this research showed partners or friends did too.
"This showed his personality is being judged on her physical characteristics. It proves the extent of obesity stigmatisation."
He added: "I think we should raise awareness of obesity discrimination."
Jackie Cox, of the support organisation The Obesity Awareness and Solutions Trust (TOAST), said: "Every fat person knows about this. Fat people live their lives will all of this all of the time.
"There is massive prejudice and stigma out there and this research shows that it rubs off on your partner."