It's simple, he's shorter than the average ww, and weighs less officially, and unofficially than the average ww. Don't get me wrong, he fits perfectly fine at ww, and he's very effective, but he is a small welterweight and usually the smaller guy in the ring. Things would be different of course if he fought a jr.ww, or JMM. I'm pretty sure he would be considered just as big or even bigger than some of his opponents if he did this.
Roach was quoted that pac came in the start of the camp at 144. I don't know where u got 155 mr wonderful