exactly so its your opinion he is a 'true' welter. I agree in the sense that he is by far more than a match for any welter. In that sense the 'secret' is out....but he is still not a true welter for me.
Cotto's sholders are broader. Pac has narrow shoulders in comparison to Cotto. In other words Cotto's frame is wider, which is why he could possibly move up to 154 without being at much of a disadvantage. Can't say the same about Pac.
If he'd only fought at his natural best he might be a 140 now or even 135. But it's really hard to tell if he demolishes his opponents, isn't it?