I have had so many parents say to to me "I don't know how my child got so big, what happened?" I always tell them that there are things that happened that they can't change. However, there a couple of very easy things they can do that will help their child start on the path to a healthy weight.
Basically, when children eat too much and are not active they can become obese, and this is especially true if they have parents who are overweight. A typical obese child is eating frequently, eating portions that are too large, drinking sugary drinks, is not active, and is spending too much time in front of television, computer, and cell phone screens. In short, their energy is not balanced.
Another way to think about why your child may be obese is to think about the things that made them at risk for obesity in the first place. For example, if a child's mother had diabetes during pregnancy, they are more likely to be obese by the time they are 5 years old. Also, did a child's weight rise rapidly after reaching its lowest point. This is called adiposity rebound or crossing growth percentiles. This means that the child got too big too fast and did not have time to get back to a healthy weight. While diabetes during pregnancy and getting too big too fast are things that are no longer in your control there are things that you can change that are in your control.
A study from 2004 tells us that there are five things that can influence whether or not a child will become obese. The number one predictor is if their parent is overweight. However, it is important to say here that many overweight parents work very hard to help their children keep to a healthy weight and are successful in doing so. The remaining influences are the child's temperament, the parents lack of concern about their child's weight, a child who will throw tantrums over food, and a child who does not get enough sleep. There are some other ideas floating out there that point to food chemicals and packaging and their influence on childhood obesity. Basically scientists are studying whether the chemicals used to grow our food are disrupting the way our children grow and develop. Also many obese children live in inner cities or suburbs where they aren't allowed to freely play by themselves or in groups.
As you can see there are a lot of reasons your child may be obese, but most likely they are obese for a combination of these reasons. Try to choose one or two possible reasons to focus on. Make a plan to help your child. For example, some doctors will suggest that you do not change the kind or amount of food your child is eating as a first step. They rather that the parent limit the total numbers of hours spent in front of any kind of screen, including cell phones to only two hours. A few doctors I have spoken to have told me that they will often see a child lose weight just by removing a child's television and computer from their bedroom. A next step may be to remove all processed foods from your home including sugary drinks.
If after a four to six weeks you do not see a decrease in weight, you should visit your doctor and ask their help in understanding why your child is obese and ask them to help you make a plan.