Simply put, when he was down at the lower weight classes he was under-nutritioned. If a fighter is training 3 hours per day, the amount of carbohydrate he would need to consume would be quite monstrous. However if a fighter trains like that but only consumes an average amount of carbohydrate the he will be able to remain at a lower weight, because his body would start to break down muscle to use as fuel.

Pacquiao won't be the only fighter who has been under-nutritioned. In fact, plenty will be.