Being "overweight" is measured in BMI, or Body Mass Index. If your body mass is out of proportion with your height, you are considered overweight.
Thing is, muscle weighs a lot more than fat. A healthy, athletic, muscular person might be classified as overweight because of their dense muscle mass.
Weighing more than average doesn't make you unhealthy. If your weight is due to muscle mass more than fat, then you're still healthy. And look at some overweight players in sports, like Prince Fielder and David Ortiz. Both are some of the strongest men in baseball, yet they are overweight.