There is little difference between the weights of 11-year-old boys and girls of similar heights. However, it is not uncommon to find height differences of 30 cm or more in children at this age thus resulting in a vast range in weight.
By comparing your child's shape with others of the same age at school you should be able to judge if your child is under or overweight. If this is something of concern it is best to consult a doctor, bearing in mind that at this age children can become very conscious of their weight.
Over the next four or five years your child is likely to grow considerably taller - up to 30 cm is possible over 18 months. In discussing weight matters with your child, you should thus be aware that they may further 'grow into' their bodies over time.
Providing your child with a good varied diet and making efforts to ensure that they get a good amount of exercise should ensure that your child is around the average weight. A varied diet should keep 'junk' food at a minimum, although the occasional visit to a burger bar or fast food outlet should be relatively harmless.
Monitoring what a child is drinking is also important in terms of overall health, particularly dental health. Some fruit juices can be harmful or make your child overly energetic, however if taken in moderation there should be no major problems or issues impacting their health or weight.