i believe that certain bodies do not wanna sanction a fight with too big a weight difference between the fighters because it makes it look like a big man savagely beating on a small man. There are rules if you read around on the net about the maximum weight difference that a sanctioning body will tolerate. For JrWW it's something like 8 lbs which means that Manny would have to weigh within 8 lbs of Margo's weight for the sanctioning body to accept the fight.
I don't regard this as a legit 154 lb title fight, but nothing to do with the weight Pac came in at. it was his forcing of Marg to weigh 151 or less. The division limit is 154 and if one fighter is not allowed to fight at the maximum weight for the division, the title win becomes tainted. Pac can come in at FW if he pleases. I feel the same way re Cotto/Pac. That said, they were both great victories. Just not for the belts.