A study involving interviews with 92 children aged four to nine found most saw a larger body as a negative thing, and weight loss as a positive.
Researchers from the universities of Leeds and Newcastle carried out the study, which involved reading children a book in which the main character had either a normal-sized or larger body.
Each child then viewed and discussed with a researcher a drawing of the character, shown to have either lost or gained weight.
If the child noticed the character's weight change, the researcher asked the child for possible causes as well as reasons for gaining or losing weight and what might happen as a result.
The study found that all children aged six and over noticed the weight change, as did 75 per cent of four- and five-year-olds.
Children were also more likely to link weight changes to food than exercise.
Some boys aged under six saw weight gain as the person "growing big and strong" but most younger children viewed weight gain as negative.
They also saw weight loss as a positive thing, but could not explain why.
Meanwhile, older children aged over six said people who put on weight could be seen as "lazy" or "greedy", and older girls said they could be ostracised by others, or suffer bullying and loneliness.
However, several children said friends would accept the person regardless of their size.
Suggested reasons for weight loss included looking good or getting fit or doing well in sports.
The authors, who presented their findings at the European Congress on Obesity in Glasgow, concluded: "Participants were knowledgeable about weight change, including causes, consequences and motivations.
"Children varied developmentally in their expressed understanding. Younger children were less able to articulate reasons for their negative opinion of weight gain.
"Older children provided numerous examples of weight bias against the larger character, and older girls particularly emphasised a loss of social capital.
"Children overwhelmingly associated a smaller body size with fitness, health and sports ability.
"Some children, however, advocated an accepting attitude towards larger body sizes."