We calculate your exercise calories using your profile statistics, and exercise effort values knows as "METs." Because your specific statistics are part of the equation, your calories burned will vary if you gain or lose weight, and will be different from other users of the site.
If any of your profile information changes, for example, if you gain or lose weight, then the calories you burn while exercising will also be affected. If you think of your exercise calories as fuel, and your body as a car, the less weight in your car, the fewer calories it will burn.
If your exercise calories have changed dramatically, please see this article for some troubleshooting steps.