When I first began taking group lessons and coming to publics once or twice a week to practice, I was really sedentary otherwise, eating badly, and at the heaviest I'd ever been. The skating initially didn't help, but I started eating better and doing other workouts and the weight began to inch off.
But when I began taking private lessons and became one of those people who was at the rink several hours a week, and started swimming on top of it and watching diet...well, about 10-15 pounds fell off pretty quickly.
I know other adults and kids who skate all the time and are either overweight or stocky. I really think it's just about the body and metabolism you inherited. Skating frequently or other exercise will help you lose some weight and build muscle tone, but if everyone in your family is heavy, you're probably not going to ever be really thin.