If you eat 500 calories LESS each day, you will lose one pound of body weight in a week. If you eat 500 calories MORE each day, you will gain one pound of body weight in a week. But these things are true ONLY if you do not increase or decrease your level of normal activity

Activity (exercise) such as weight training/bodybuilding burns between
200 and 450 calories per hour depending on your weight and intensity.

A mars bar contains about 448 calories!

Do the maths! 

Train Heavy

John