That simply isn't true. Unlike running, cycling at an low endurance pace is quite gentle on your body. Reaching 500kcal/session isn't going to be a problem after a few weeks of getting your mind and body accustomed to biking for more than 15-30 minutes, and chances of injuries will be quite low, especially if you use a stationary bike. You can hit 400kcal in an hour of biking without too many issues. Larger people will generally put out more power and therefore burn more calories.