These little guys always blow up in weight, cutting down to such low weights is not healthy for the body so when they start getting normal amount of calories and carbs their body blows up.
Its not healthy to cut weight its why a lot of fighters are screwing with their system and metabolism like that.
If you notice the fighters who fight closer to their normal weight and stay in shape year round never balloon, Bernard Hopkins, Manny Pacquiao, Juan Manuel Marquez, Floyd Mayweather these guys all fight at their walk around weight, meaning they only cut ten pounds or less aka sweating it off in the sauna type cut.
Sugar Ray Leonard is almost 70 and the guy still walks around at 168 pounds.