Think of pitch as the level something has been set at, measured from the bottom up. A singer sets a note low on the scale and you hear a deep sound. A builder sets a roof low on its angle and you see a gentle slope. In both cases low-pitched means the same thing: this has been set near the low end, not the high end. Nothing here is about size or loudness, only about where on a range the thing sits.
可以想成 pitch 是「被設在哪個高度」，從底下往上量。歌手把音設在低的位置，你就聽到低沉的聲音。蓋房子的人把屋頂的角度設得低，你就看到平緩的斜面。兩邊的 low-pitched 其實在講同一件事：被放在靠近低的那一端。這裡跟大小聲、跟房子高不高都沒有關係，只跟「落在範圍的哪裡」有關。