Of course speed is a reason but most surprisingly, I think the real reason is strength. I think that's why a fight between the two is so interesting. Guys like Mosley, Cotto, Hatton, ODLH, etc... all these guys at welterweight worry about making weight. Instead of them putting on muscle to gain strength and speed at the same time, a lot of these guys are only running and sparring to lose weight. Mayweather and Manny are both guys that walk around below or near the welterweight limit very comfortably. They can do all their running and sparring but they can also weight lift to get stronger and more explosive while keeping there speed. If you notice, Mosley was the naturally bigger guy on Saturday but he wasn't as strong and sturdy as Mayweather. Mayweather was able to match and even in some cases beat Mosley's strength even though he's a smaller man than Mosley. It's because Mosley may be naturally heavier but that doesn't mean he's as physically strong. Same with Pacquiao, Cotto's a naturally larger man, but Pacquiao's being able to put on lean muscle creates strength, punching power, and explosiveness. Both Mayweather and Pacquiao have room to put on muscle and that coupled with their skill and condition is allowing them to completely DOMINATE the welterweight division despite being smaller guys IMO.