People who are overweight are seen as "too fat" to commit crimes, research suggests.
A study found those of muscular build were regarded as the most likely to commit crimes, followed by people of normal weight.
Just 10% of those who were overweight were regarded as most likely to commit a crime.
Dr Julia Shaw, senior lecturer in criminology at London South Bank University, who carried out the study, said: "Stereotypes about body type may be getting in the way of justice.
"Our findings suggest that if you are an innocent suspect of a crime but happen to be muscular, you may be at a significant disadvantage.
"Body type is often overlooked as a basis for discrimination, and has rarely been examined in legal contexts.
"The present research is evidence that the body type of a suspect can impact the partiality of eye-witness accounts."
The research on 93 people was published in the journal Psychiatry, Psychology and Law.
The participants were shown one of two short videos, depicting a violent or non-violent robbery.
In one video, the victim had their wallet stolen, and then was violently assaulted and knocked to the ground.
The other video involved a non-violent crime where the victim's wallet was stolen without them noticing.
The thief was dressed in a way that concealed their body and face.
People were then asked to identify the thief from a suspect line-up of muscular, overweight and normal suspects.
The photos in the line-up had been previously altered so that each face in the line-up was randomly assigned a body-type.
The true thief was not offered as a suspect choice in the line-up. This meant the participants in the study had to rely on their own judgements and bias to choose suspects.
The results found that 51% of participants accused muscular suspects of being the perpetrator in the video.
Some 37% thought suspects of a normal weight were most likely to have committed the crime. Just 10% believed overweight suspects could be the aggressor.
Dr Shaw said: "It's surprising that this was the outcome for both of our scenarios, a violent mugging and a simple theft.
"We may have expected that muscular body types may have played a bigger role in the scenario involving a physical altercation, but this was not the case.
"Muscular men were always at a disadvantage compared to individuals of other statures, regardless of the type of crime.
"In practice this means that police may benefit from ensuring that suspects in police line-ups are matched as much as possible on body type to avoid relying on stereotypes."