Should I always lift heavy weights to get big muscles?

No, increasing muscle size, or hypertrophy, is dependent on several factors, and the amount of weight you lift is only one of those factors. Getting your body to grow is simply an adaptation it will make to the demands being placed on it. If you only lift heavy weights, you may become good at lifting the heavy weights and develop higher levels of strength, but this will produce little results in terms of muscle growth. One important aspect of getting bigger is nutrition. In order to grow your muscle size, you must consume more calories than you are burning. These extra calories will help provide your body with the energy necessary to build the muscle. You must be sure the additional calories you are consuming are nutrient dense, such as protein and carbohydrates, as opposed to energy dense, such as fat. Although your body does need fat for many important functions, it is easier for your body to store fat. It is recommended to take in 1.6-2.0 grams of protein per kilogram of bodyweight per day and 50%-70% of your total calories should be made up of good carbohydrates, such as whole oats and grains. Fat should make up 10%-30% of your total calorie intake. Increasing your muscle size requires your exercise program to consist of higher volumes of training (3 to 5 sets of 6 to 12 repetitions) and intensities of 75%-85% of your 1 repetition maximum. During the training session, long rest periods are not recommended.

I guess this depends on what you consider heavy weights. The most common intensity range used to increase muscle size (hypertrophy) is between 75%-85% of your 1 repetition maximum. This equates to being limited to lifting weights for a maximum of 6-12 repetitions. This may be considered heavy weights by some, however this is often consider a “moderate” weight. Below is a list of the variables (provided by the National Academy of Sports Medicine) that may best increase muscle size:

                SETS:                     3-5

                REPS:                     6-12

                Intensity:                75%-85% of 1RM

                Tempo:                   2 second lift / 2 second lower

                Rest:                      0-90 seconds

                Duration:                4-12 weeks

                Frequency:              3-6x/week

There is also a very important nutritional component. Protein, and overall calorie intake, must be increased. A simple calculation to know how much protein you should take in is to divide your bodyweight by 2. This is approximately how many grams of protein you should have daily. You should also make sure you increase your vitamins, minerals, and antioxidants intake during this intense training in order for your body to recover, heal, and grow.