Why does weight class matter in Pac-JMM? It doesn't! Evidence.

No...you are getting confused.

If JMM weighs in at 135, and fights at 145, he would be at a huge disadvantage against Floyd because Floyd would come in at 147 and weigh in 154+.

Now if JMM asks for Floyd to come down to 143 like he did, Floyd would now have to dehydrate to 143, weakening him, and depending on Floyds body composition, may have to lose muscle too. So if Floyds natural weight is 146lbs+7lbs of water = 153lbs fight night weight, if he had to go down to 143lbs, he may have to drop 8lbs of water further dehydrating him to get to 145lbs + lose 2lb of muscle to get to 143lb. This will balance things out.

JMM also knowing he was gonna be outweighed, tried to put some extra weight on, or didnt bother loosing the extra fat in training camp thus he looked slower. His logic was, if I come in at 144lbs at fight night naturally (fully hydrated) then I have to carry an extra 5-6lb of actual weight(not water weight) but fat or muscle.

Question, why would he need to do the above against Pac?

Oh, why have the 144 for Pac? Well if he has it at 147, Pac could very well bulk to 154lb, dehydrate himself to 147 and then fight at 153. This wouldnt be too bad for Pac as he would only need to cut 6lbs of water. JMM would then be at a big disadvantage. At 144 it limits it a bit, Pac could bulk up to 153, but then he needs to dehydrate himself to 144 which will hinder his performance.

I dont think Pac wants to dehydrate anymore though so unlikely he will try either. Most likely he will come into the fight 148/149. JMM at 146/147 which means JMM bulked up by 1-2lb.