I always say that every body has a physique with equal number of muscles, but every one can never become a professional bodybuilder. Yes , every one can improve up to certain level with regular training, sensible eating and with optimum rest. In spite of all the best training, diet and schedule or even with drugs, one can never improve to unexpected level if he is not blessed with great potentials. Bulk up means adding pounds of weight on your frame with mostly fat and some muscle with the help of expensive diet, supplements and with hours of training to an out of shape physique.
When the competition comes in, then the real preparations
The off season should never be used for bulking up, rather it should be used to strengthen your weak parts and keeping your strong parts in best shape by not indulging in eating senselessly and wasting time and money. You should stay lean throughout the year, just 2-3 kg near to your competitive weight and 2-3% body fat close to your contest percentage. And keep getting stronger and bigger but not increasing your body weight. BODYBUILDING IS GETTING STRONGER, BIGGER, LEANER AND LIGHTER. LOOK BIGGER IN LOWER WEIGHT CLASS.
I may tell you, that you can be professional bodybuilder of national level only if your have good muscle mass and your normal low body fat percentage is in off season. The body fat percentage is less than 12% for national level and it should be less than 8% to be an international level with extraordinary muscle mass, proportion and shape. Stay cool and keep adding muscles every year but not getting much heavier to your natural body type, fame and potential.