I don't think the weight is as much of an issue as the body stress. as others mentioned you don't go from couch to straight running no matter what you weigh. Your body simply isn't used to it. I'll speak from personal experience here. When I started back in the gym last year heavily I had issues with running at first. some people may remember my long thread about shin splints. Long story short I was pushing myself too hard and hurting myself. Now i can run 3-5 miles at a 7.5 mph clip 3 times a week. You just need to ramp up and listen to your body.