If you want to bulk up or increase your muscle mass, then you need to focus on a certain way of lifting, and that is called, The Overload Principle. You always want a challenging resistance load on the muscle in order for it to increase in size. For example, a typical overload principle routine would be reps of 12-12-10-8-6 and then maybe a burnout set at the end of 15 to 20 reps. The most important part of this principle is that with every set you increase your weight by at least 5%. If you keep your weight the same the entire time, willl get some hypertrophy gains in the beginning, but you wont get anymore because you are not challenging the muscles on a consistent basis.
Best of Luck & Happy Holdays,