This is true, but you CAN add mass without increasing size much, because there are two ways to add strength. One is to increase the number of fibers in a muscle, 'growing' it, and another is to increase teh fiber density, which would not 'grow' it. Your strength goes up in both cases but you will only see large size gains if you are increasing the total number of fibers, not the fiber density.
I did kind of point that out. But I went and asked my physiology professor about muscle fibers/growth, and her response was
"Muscle growth is complex and is still not fully understood but it is believed that the number of muscle fibers is fixed and they increase in size with exercise."
So I'm not arguing or disagreeing with you except about the statement,
One is to increase the number of fibers in a muscle
And for that matter I wasn't entirely sure either which is why I asked my teacher.