Pacquiao's giving up a lot of size but I think it's just manageable. Manny's chin held up quite well against power shots that he saw coming (As opposed to the Marquez counter) but Mugabi would be the hardest hitter he ever faced by a considerable margin. Mugabi would have to stop him since the activity level of Pacquiao would rack up points and after round 8-9ish Manny is going to be winning rounds for free because of the difference in conditioning. Pacquiao on points, wouldn't bet money on it.
Mugabi gave Hagler trouble because he was bigger than him, Mugabi was not a natural light middle he was huge and would destroy Manny and anyone Manny ever fought, far too big and power, his reach would be too much, Margharito did not have Mugabi's power and would have been finished early by a prime Hagler.
Pac never swayed on how much pain he was in from Margarito barely hitting him. I thought he was just saying it after the fight but even well into his forties he was saying how he was hurting in bed the next day and knew he'd never go that big again with his opponents. And that's only a Margarito-level puncher, who was being watched VERY carefully for glove issues, I'd assume. A faded Margarito just above welter at a catch weight. I think it's just too much for him too.