It's a very serious problem that the charity, Weight Concern, is trying to help fix.
They're concerned because being overweight is very bad for your health, both physically and mentally.
Many overweight children suffer a great deal from teasing and there is concern that if children are continually bullied, they could develop eating disorders.
Anxiety and stress
One psychologist said: "Children are teased not only by their friends but by adults around them, by family members and by teachers at school."
"Children who are teased about their weight suffer from anxiety and stress and in extreme cases can be driven to suicide."
Weight Concern hopes to be able to reduce the problem in this country by promoting a more healthy lifestyle and way of eating.